I have used Rolleiflex T (with Tessar 75/3.5 lens) for about 2 years and only few months ago I finally decided to sell it and get Mamiya 6 .
When I first got the Rolleiflex I got to like it very quickly - the camera is well done, purely mechanical. The way one uses the camera is very different compared to SLR. you are looking down while focusing/composing, the image is left-right mirrored. It takes a while to learn how to move the camera to level it or get a different view. I really liked the coupled shutter/aperture lever on the T. The camera directly showed EI values like a light meter, so it was very fast to set it to correct shutter/aperture combination after you took the reading with the light meter. Of course it is possible to de-couple the setting of the shutter and aperture values.
The lens on the T was great for portraits at around f/4 to f/5.6 and got really sharp stopped down. The camera is rather light and robust - not much to go wrong. If you need to focus closer than 1 m you can buy rather cheaply a set of dedicated close up lenses (consists of 2 close up lenses and 1 parallax correcting lens for the viewing lens) in 3 strengths I, II, III. I sued the I quite often.
I loved the automatic film-loading feature - the camera automatically notices where the film starts. And it really works. The shutter is relatively quiet, but not as quiet as the shutter of the Mamiya 6.
One great advantage for travel is, that for most cases when you need a tripod - a small table top (Manfrotto 709B is great for this - small and strong) tripod does the job even if you put it on the ground. Because you are viewing the the top there is no need to to lie flat on the ground, get dirty and look like an idiot
. Also the tripod mounting point is pretty much exactly in the center so even a lightweight or not very strong tripod can be used to hold the camera. There are no vibrations from the shutter of course.
I used the camera also with studio flashes a few times using the X-sync connection. Worked perfectly too.
Comparing the Rolleiflex T to Mamiya 6 - sure the Mamiya 6 has sharper lenses, but for enlargements up to A3 there is no difference to be noticed. Still the Rolleiflex was more joy to use, people liked it too (even if you photograph somebody - you are not pointing a camera to his/her face - much less aggressive).
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For other Rolleiflex models - there are Planars and Xenotars (performance is the same) that are either F/2.8 or F/3.5. These both should be a bit sharper wide open than the Tessar. The F/2.8 models are more pricey than the F/3.5. There is also rather ignored Minolta Autocord that also sports a Tessar 75/3.5 lens. I have read very high praise on that lens. The camera is supposed to be well made too.
I would say - if you will like using TLR, you will love the Rolleiflex independently of the model.
EDIT: The main difference compared to other MF systems is the fixed lens. TLR is not really a "system" camera. The prices vary a lot - I payed for the "T" about 200 euro (that was on the cheap side), but invested in some accessories and CLA later on. $400 - $600 could be a reasonable price for "T" in a good shape with the case. The "F" models, in particular the f/2.8 in a good shape go beyond $1000. Minolta Autocord in a good shape around $300. Watch the eBay for a while to get a better idea.
Keep in mind - that there is a good chance that the camera will need a CLA. That would be around $200 I guess (depending on what is needed to check or adjust). I payed around 160 euro for shutter CLA in Germany.