I see that you re seriously interested in BMWs!
I like the R75/6 a lot, I'm also biased as it's like my own bike
It looks very clean from the pictures and it must be the result of a professional rebuild.
The onlky drawbacks I see that could reduce the price are the colour which is not in the BMW range and the front disc which doesn't look to be standard BMW. It should be perforated like the R90S. I don't know the priced for such BMWs in US but it seems reasonable as the labour and parts are expensive and it seems a lot of work has been done on this one. Just make sure you see the bills so that you know what was done.
I don't like this R90S at all! It has many mods that can be used to lower the price, but even if they are mods made by a long lasting owner to improve some thing the bike looks bad. The wheels are from a Yamaha XS750 shaft triple as far as I can tell! They are certainly not BMW wheels. I wonder how they mated the XS rear wheel AND bevel drive with the BMW shaft
Maybe the previous owner was desperate to have tubeless wheels??
The seat unit looks to me like a /7, there is a bar linking the frame from front downtube to above the swingarm that I have never seen before (maybe to strengthen the frame??), and it has Koni shocks (that's not bad). The exhaust are not correct either.
The price difference also reflects the relative condition of the two bikes. The R90S is much rarer and more sought after, and if it was in a similar condition to the R75 it wouldn't be 500$ more, it would be something like 1000$ more!
With these bike, it's best to buy the best you can afford, because you'll end up paying much more later!
I hope this info is useful to you. Let me know if you need more details, I will help as much as I can