The V500 is similar in scanning results to the V600. However, the V600 scans at one setup time: (3) 6x7 negatives. The V500 only scans: (1) 6x7 or (2) 6x6 negatives at one time. The V500 slows down scanning time and requires more work to set up because of this limitation. Also, if youre scanning photos, ICE that reduces spots, creases, scratches, etc can be applied with the V600 but not with the V500. ICE is however available for film scanning on both models.
I have a v500. I think the main difference from the v600 is the film holders, as Alan points out. However, if you invest another $80 or so, you can purchase the aftermarket Betterscanning holder for the v500, which allows you to scan an entire strip of 3 6x6x or 4 6x4.5 or two 6x7 at once. The betterscanning holder also has anti Newton Ring glass to hold negatives flat and adjustable height. The adjustable height feature is critical. It can make as much difference in the resolution as stepping up from the v500 to the v750. For some samples of these two scanners, see my other thread in this forum:
http://www.dpug.org/forums/f6/comparison-some-flatbeds-holders-3525/
For 35mm, yeah, the limited resolution of flatbed scanners really begins to eat at performance. In addition, I don't believe Betterscanning makes a 35mm holder for the v series Epsons. I think you can buy anti Newton ring glass though, and you could prop up the Epson 35mm holder on thin pieces of cardboard to find the optimal height.
For at least two of your purposes, web display and a substitute for contact prints, a flatbed is fine. I would say you can obtain good 35mm scans up to about 2400x1600 pixels, which may be suitable for printing in a book if you don't make a huge image (i.e. don't use a scan from a 35mm for a two-page full-bleed spread).
Here's a low res web image from a v500 scan of a 35mm colour negative.
https://www.flickr.com/photos/hsandler/12645803185/in/set-72157632891133869
I have used Silverfast, but not Vuescan; however, I prefer good old Epson scan software, because it's simple and I understand everything it's doing. The main thing is to scan in 16 bits per channel and avoid clipping black and white points. After that, you can make all adjustments as well or better in Photoshop than in scan software.