I own a V750, but unfortunately have never been very impressed with it -- especially for the price paid. Maybe it's because I owned a 1680, which was really well built; the V750 seems quite flimsy and picky in comparison.
I was recently traveling abroad and was able to use a 4990 for a few weeks. I was extremely impressed, especially for its price (about $150). The 4990 image quality seemed at least as good as my V750 (although as I didn't have them side by side, I couldn't do a real comparison). The medium-format film holder on the 4990 is notably better, although it's limited to only one strip, rather than the two on the V750.
Note that my usage may be different than yours: I have an Imacon Photo which I use for final scans; the flatbeds are basically "proofing" scanners, which do a good job reasonably quickly.