I can only speak for my own experience, but here it is.
I started with a Canon 8800F flatbed, which I still have. Pros: cheap, reasonably fast (I've heard more recent models are faster), relatively simple software (though limited; many prefer VueScan); Cons: B&W images require lots of massaging to get where I'm happy with them, 35mm color results are unacceptable to me, 120 color is marginal. If all you're shooting is B&W, not a bad choice as long as you don't mind some post-processing.
Recently I added a PrimeFilm XA (Reflecta RPS 10 M). Pros: generally much better results straight out of the scanner, especially with color (colors are often flat but easily adjusted later), automatic feed (which is great when it works, which is not always); Cons: slow, lousy software (both the in-house software and the included SilverFast, both of which were designed by sadists), 35mm only, noisy, and of apparently dubious quality (it makes strange noises and I'm always expecting it to grind to a halt or try to consume itself).
I use the Canon for B&W, since I've worked out a post-processing routine that gets me grain-sharpness, or a reasonable facsimile thereof, even with 35mm (this is for traditional grain films; I have little experience with T-grain emulsions). 120 obviously goes in the flatbed as well; color can be made acceptable, but it takes some doing. Good results from 35mm color are easy in the PrimeFilm; I could use it for B&W as well, but I know how to get good results from the Canon at this point and it's faster.
It's taken years to learn as much as I have, and there's still a lot I'm missing, I'm sure. A dedicated film scanner will, all things considered, get you far closer to a finished product than a flatbed. They cost more, of course, and they have their own drawbacks as well (especially speed).