There's a possibility of problems with the old PS2, no matter where you get it from. If you do have problems, you can just return it and get a replacement (of course, there's cost of shipping).
And if you get it from GS, you won't get any manuals or anything. If you ever traded in a system with box/manuals there, you'll see that they just throw away the box/manuals- don't know why.
The new PStwo (PS2slim) is problem free (so far). I haven't heard anyone complaint about it. Of course, most old PS2s run fine most of the time, except for a disc error now and then. The more common problem is slow drive speeds but that's doesn't effect whether a game works or not.
It's really up to you as if that extra $40 is worth it or not. If you're not strapped for cash and have lots of PS2 games to play, I say go with the PStwo.