If you want to use the roll film back you will have far more choices for far less money than in sheet film. The same is even true, though to a lesser extent, in black and white.
So we really have two questions here - whether to start with sheet film or roll film, and then which film.
Advantages of using roll film in the roll film holder:
1. No darkroom or changing bag needed to load and unload film - may or may not matter to you.
2. Far lower film costs, allowing you to learn about film and the movements and use of your view camera without spending nearly as much money.
3. Higher shot capacity. You don't say how many 4x5 film holders you have, but carrying, say, 10 holders, which hold 20 sheets of film, is fairly bulky (and also costs about $50-$60 to load with color film, give or take.) The roll film holder will give you 10 (6x7 cm) or 9 (6x9 cm) exposures per roll, and you can carry a bag full of rolls if you want and change in the field, at a cost per shot of about $0.50 versus $2.50 or more per shot in 4x5.
4. No, or very few, dust problems. I'm assuming you've not used a view camera before so the advantage here may not be obvious but, trust me, it's a good one. I seldom have dust on my sheet film but I'm fanatical about how I load my holders, and I do it in my darkroom with the table wiped down and the HEPA air cleaner running. Especially for those who have to use a changing bag, it can be really annoying.
So just use roll film? Well, not quite.
Disadvantages of using the roll film back versus sheet film:
1. Limited ability to use wide angle lenses. You don't say what lenses you have but this may be important (or not, if you aren't much into wide angles.) The widest lens I have for my view camera is a 90mm, which on 4x5 is pretty wide, roughly equivalent to a 28mm on 35mm film. In 6x7 that's a dead-normal lens, maybe just slightly wide on 6x9. There are, of course, wider lenses you can get and use, but even so, if they cover 4x5 they are often expensive and not so common, and if they don't cover 4x5 their usefulness is limited to just your RF holder. The front and rear standards are going to be pretty close together when using a wide lens, and unless you have a bag bellows the binding of the bellows can severely limit your available movements. And even if you do have a bag bellows it can mean changing the bellows when changing lenses - not a huge thing but one more thing to carry and fool with.
2. It's really kind of, well, fiddly at best, using a camera designed for 4x5 as a medium format view camera. I got a roll film holder myself thinking to use it for color since I mainly shoot black and white in 4x5 and color is so expensive, but I use it less than I expected. Partly this is the lens issue above. But it's just harder to see what your movements are doing on the ground glass when you are using such a smaller part of it. Of course, with most medium format cameras you wouldn't even have movements.
3. Quality - this isn't as big of a difference as it used to be because modern films are so good. It depends on your output size and media, but medium format produces superb prints of at least 16x20. You may not notice any real worthwhile difference, but all other things being equal and done correctly, 4x5 will still have the edge. The point of a big camera is a big negative!
4. Film selection by shot: this is more important by far in black and white, because you can develop each sheet to suit the individual scene. It's less important in color but still an advantage for sheets. I'm getting to my recommendations between two films, one for bright, pretty much exaggerated colors when that's what you want and another for normal looking color and great skin tones. With roll film whatever you have loaded in the holder is what you shoot, unless you have two holders - or you can shoot an entire roll on each subject (gets expensive and otherwise unnecessary) or just wind out the rest of a partial roll (wasteful) to change. Unlike 35mm there is no practical way I'm aware of to change film mid-roll without wasting the rest of the first roll. So if you want to carry both the bright colors film (Ektar) and the normal film (Portra) and shoot whichever fits your subject, it's easy to load some holders with each and just expose as you like for each shot, or even both for the same shot to get both kinds.
Those are my thoughts, anyway. Now on to the film.
Assuming you want to shoot sheets some or eventually, it seems best to select a film that's available in both 120 and sheets. That way you only have one film to learn regardless of which format you use.
Negatives or transparencies? Some people love transparencies, and they sure do look impressive on a light box. But aside from that I see little reason to shoot them these days. Exposure is a lot more critical, and output media are more limited. With Ilfochrome gone the only ways to get a print are hybrid, either via scan and inkjet or scan and lightjet or similar, or internegatives. With the availability of Lightjet I don't know anyone doing internegs commercially and not many people do them at home so that pretty much leaves hybrid. That's fine if that's what you want, but you give up nothing with modern negative films in that regard either. Or at least I have received back scans from my negatives that are just as good as those I've gotten from my transparencies. I'm not scanning myself yet so I can't really say much more. Modern C41 films are intended to be scanned but E6 may still be easier to get good scans. I have some 120 and 4x5 E6 film in my freezer wondering what I'm going to do with it now that Ilfochrome is gone. I guess I'll shoot it and output via hybrid methods, but I'm an analog photographer and that's not really my "thing." (I picked it up at a good price before the Ilfochrome announcement, intending to get back into Ilfochrome.)
The only reason I really shoot transparencies any more is for projection, and for that I shoot 35mm.
So my suggestion is color neg, which gives you the same hybrid output options as transparency film plus easy (well, relatively - you can do it yourself in the darkroom or hire it done some places still) optical printing.
You don't say where you are. Fuji is good film, but in 4x5 it's almost impossible to find Fuji color neg in North America, at least without paying astronomical prices to import it yourself. So Kodak is it. Fortunately, Kodak makes not only some of the best negative film available but some of the best negative film ever available.
You have, for practical purposes, three choices: Portra 160, Portra 400, and Ektar 100. I use the latter two.
For striking color saturation use Ektar. For everything else, use Portra. Ektar is great for dull light to make the colors pop, but in already strong light it can be too much, but that depends on your taste in color and contrast. Portra is great film for practically everything, though I use Ektar for really bright colored landscapes and such.
Portra 400 is superb and has finer grain than you will ever really need in either 4x5 or medium format. The extra speed is handy, as view camera lenses tend to be slow by the standards of 35mm and medium format and furthermore are usually not very good wide open (unless you're going for the shallow DOF and don't care for ultimate sharpness as in some portraits in which case they excel.) So this has all been a very long winded way of me recommending Portra 400, with maybe a bit of Ektar in your kit to play with!
