There's not really a lot of difference. Here's a comparison of the two: http://www.food-dehydrator.com/resource-center/food-dehydrator-reviews-nesco-fd-75pr-versus-nesco-fd-80.aspx
As far as I know, the primary difference is the FD80 is square shaped and the FD75 is round, and the trays on the FD80 have more vertical space. I bought the FD80 because I like the square shape more and also because the trays are taller (2.2 in vs. .5 in according to the above comparison) so they should have more vertical space for fatter peppers and/or whole peppers . I was a little worried that there might not be enough vertical space for drying whole peppers or bigger things with the FD75, but I've never used an FD75 or seen one in person so I don't know if that's the case. The FD80 is newer, has a little more drying area on the initial trays, and is actually a little cheaper on amazon.com (and most other places it seems). On the other hand, the FD75 supports up to 12 trays, while the FD only goes up to 8, at least according to the manufacture. There's nothing to stop you from stacking as many trays as you feel like, above and beyond the 12 or 8, so I'm guessing there must be a significant decrease in performance beyond those points. The fan probably doesn't penetrate effectively beyond those numbers and/or it doesn't maintain the right temperature. It's not really an issue with peppers or most foods, but I also noticed with beef jerky that the corners of the machine don't seem to dry as well as the other areas with the FD80 as well. So the circular design of the FD75 might do a more even job of drying.
I got the FD80 for about $50 with free shipping on buy.com last year, then bought another 4 trays from everythingkitchens for about $20 if I remember correctly, and I'm very happy with it. I'm excited for mushroom hunting season this year, I can dehydrate all the ones I find and use them throughout the year. It will be good for drying mulberries and blackberries and other fruits this summer that I can pick pounds of and then reconstitute and use to make wine later.
A timer would be nice, but I doubt I would really use it that much. You could just get a socket timer from a hardware store for $4-5 and plug the dehydrator into it if you really need a timer.