First off, the 17" Machine is not a Mid 2010; they might have bought it right before the actual Mid 2010 machines came out (and if it still has AppleCare for another month or two, that might be something to consider as you could essentially take it in to get further looked at before the warranty finally expires). At best, that's a stock Mid 2009 model; still a fine machine.
Having said that much, the 15" that you have there was the only 15" MacBook Pro that DIDN'T have a discrete graphics card and was essentially, internally, the same machine as the high-end 13" MacBook Pro of that same generation, but with a bigger screen and an audio-in port. You do have the high-res anti-glare and that is nice, but given that you can buy fairly decent anti-glare films to put over the glass, I'd say it's a benefit that only goes so far.
As for that 17" MacBook Pro, you are paying quite a bit more, but it's quite a lot better of a machine. In 2013, I don't know that I'd put much money towards a Core 2 Duo Mac UNLESS it was a high-end 15" or 17" MacBook Pro from 2009, or a low-end 13" MacBook Pro from the actual Mid 2010 generation. Even then, I'd be sparing. But given the screen, the graphics, and I'm guessing, the good condition (since you noted no cosmetic defects), I'd go for the 17". Though, I'd try to drive the price down on grounds that it's not the Mid 2010 generation, and thusly, it should be worth quite a bit less than 900. If it actually was the Mid 2010 generation, and it thusly had a dual-core Arrandale Core i5 or i7 in it, then it might be worth that price.
They're both Mid 2009 machines, the 17" isn't newer, it's just better spec'ed.