The biggest difference in perceived grain you will see between film scans has to do with the smoothness of the emulsion side of the film and the nature of the scan light used.
Some films have a "rough" emulsion side. If you shine a light on these, you'll see a kind of "frosty" look in that side.
These will scan with excessive apparent grain in any scanner with a "hard" light source, such as most dedicated film scanners.
The phenomena is called "scan aliasing" and is the cause of many of the claims of "large film grain" in the past. It is not true film grain. It is just the scanner catching the roughness of the emulsion side and making it look like grain. There are a few websites that go to great lengths explaining what it is and why it happens, no need to go into it here.
Flatbed scanners usually have a softer light source, typically like a fluorescent light. These tend to generate a lot less scan-aliasing "grain".
In the days of widespread use of enlargers, you used to see a similar effect between hard-light condenser enlarger heads and softer ones, usually called "cold-cathode". Similar effect and similar results. Back in those days ALL films had a very rough emulsion side, so the problem was a lot more visible. Cold-cathode enlarger heads were great to minimize surface defects on film such as scratches. Same reasons.
Most modern -by that I mean batches less than 3 years old - colour negative films have the emulsion side "shiny". To the point where it becomes nearly impossible to discern which is the emulsion side, unless one uses the markings on the edges of the film to establish which is which. These films scan with almost no scanner-generated "grain".
Typical examples: any of the latest batches of Fuji Superia and Xtra, Fuji NPS and NPC films. Also Kodak BWC400N (b&w chromogenic) and the latest Ilford XP2 Super batches. Most of the remaining Kodak colour negative films are like that as well, at least the latest batches. I don't know about the new Ektar, haven't seen it yet in Australia. But I suspect it will have a very smooth emulsion side.
In the colour reversal films - colour slides - you get the latest Fuji Velvia 50, very recent batches of Astia and the very recent Provia 400X. Kodak Ektachromes are also very smooth and not prone to this problem.
Most traditional b&w films and older colour films are almost without exception in the "rough emulsion" camp. They need special post-processing when scanned with hard light sources, or the use of a flatbed scanner with a "soft" tube light source.
