Lotus, a lot of this is my opinion, so take that for what its worth, but I'll hit the important points that jump out at me.
The silver bike at Gateway is one of the three original colors for these bikes. To me that's a plus, even if it turns out to be a repaint. I understand Gateway has a good reputation, but someone else will have to jump in on this opinion. The bike looks well cared for just from the long distance pics. I bought mine at around 34K miles and it was well cared for. I had some transmission issues later on that were solely caused by a previous owner up north parking the bike for a really long time with water in the transmission oil. (Documented the fun in my Tale of Two Transmissions thread on here about 1 1/2 years ago). Other than my misfortune, that bike at that mileage shouldn't have any major issues coming up. The usual electric stuff can crap out from age - switches, relays etc., but that can happen to newer bikes just as easily.
The other bike... well some of the facts in the sellers ad don't jive with the R80 ST history as I have read about it. Production numbers are off; I'm not sure about the number quoted as total US imports. That kind of inaccuracy red-flags an ad for me. The green color is not a stock color. (They were sold in red, silver, and gray - yeah, seems odd). The unknown mileage isn't a big problem if it is because the speedo cable was broken for a while. But if the speedo or odometer is broken, that may cost you some $$.
The silver bike still has the air injection plumbing on it, while the green bike has had it removed. If you are not familiar with the ramifications, the short version is the air injection supposedly caused higher temps around the exhaust valve which could hasten valve seat recession. (Mine still had it when I bought at 34K miles. No valve seat recession, but I did remove the plumbing and stored it away).
For either bike, they look pretty clean. Also for either one, dig into the condition of the exhaust system. The stock muffler and pre-muffler, or collector box are NLA from BMW. The header pipes still were last time I checked. The good news is that Keihan in England makes identical replicas in stainless steel that are of good quality and some of the pieces are actually cheaper than the OEM stuff. I got lucky and got a great deal on a blemished Staintune R100 GS muffler in stainless that bolts right up like the stock muffler, but down the road I will be ordering Keihan headers and collector box for my ST.
If you are new to airheads, spend some time with a big cup of caffiene and read through Snowbum's site - really good expertise for free. http://bmwmotorcycletech.info/techindex.htm
That's enough of my ideas... Next! ...