I went back to the first game after beating Going Commando and the only minuses that I noticed was the controls didn't feel quite as tight and Ratchet can't strafe, which I used extensively in the sequel. Of course the second added more to everything, but if you play the original first, you won't notice. If you get the chance, play both, but if your only going to play one, I'd play the sequel. GC gets about as perfect as an action platformer can be, IMHO.
I also believe that if you play through the first game, you can unlock certain weapons in the second game right away. I'm not 100% positive, but there is a reason to get through the R&C game first before Going Commando.
There is a vendor on one of the worlds in R&C:GC that will sell you some of the weapons you used in the first game. If you've played the first game and acquired them the weapons will be free, otherwise you'll have to pay for them. Honestly, I didn't find them to be very useful at their base power (it's better when they get mega ammo), but it's still a cool addition. [/quote]