There are obvious differences in what you are paying for when comparing the 600, 700 and 750. I will do my best to focus on things others did not mention.
The 700 and 750 both have a dual-lens setup, each tuned for a different resolution. The 750 gets improved optics (coatings) and a fancy "high-reflection" mirror.
While the hardware resolutions report out the same, the 700/750 offer a better motor which likely also improves the reliability. The reliability for the 600 is quoted at 30K cycles, while the 700/750 is 100K cycles.
The light source in the 600 is LED, while the 700/750 use a white cold cathode florescent bulb. People argue about this kind of thing.
Ken Rockwell would tell you not to worry about it.
Based on the specs the 750 scans twice as fast as the 600. The measurements are in ms, but faster is faster so I guess it depends on what you need to do with your time.
Finally, the 750 comes with a fluid mount adapter. It doesn't come with any of the tools you use to wet mount, but it is a nice addition if you are so inclined.
One could argue the relative value between the 700 and 750, since not everyone needs all the extra features: upgraded versions of Silverfast, the Monico EZ color kit, the lens coatings, the wet mount adapter etc. I think its safe to say that the 600 is in a different class pulling in less professional features and overall lower reliability. That said, it is cheaper by a lot and if your budget happens to be making the decision then maybe it will do.
I use an Epson V750 for wet scanning large format film. When I am working with 35mm or 120, I wet scan with a Nikon 9000. I agree with Philip, a dedicated film scanner is no match for a flatbed. That said the V750 is a lot cheaper than a Hasselblad or drum scanner and I appreciate the quality I get from my scans. When I look at my own workflow, I could have gotten the 700 and missed out on the nicer optics and the Monico EZ color bits, but I decided to just take the plunge. I do not regret my purchase.
Consider your expectations, since ultimately you know what your time, money and images are worth. Scanning at the highest possible resolution to archive film seems like a one time thing and something you could outsource to a lab. Yes, it is more expensive (really expensive if you need the highest resolution), but it will take up less of your time. If you are pursuing the scanning project for the love of the images then maybe your time will be worth it, but then consider what you plan to do with the images once they are digitized. How much resolution is required - bigger isn't always better. Finally if there is some commercial significance (e.g. you stand to make money from selling the images) then I wouldn't be cheap on the scanner since its the thing besides you and the image that is making you money.
Hope this helps. Good luck!