Carl -
I spent 5 years trading options in NY, and have friends who still do the same. I also have friends at the I-banks. I would say that, out of 10 friends in each field, only 1 of the traders is now making as much (and he has made a lot more) as the bankers. It's as simple as that. Trading is hard, and there are a lot of variables in trading over which you will have very little control.
You're obviously a smart guy, and if you did well during your summer position and received an offer, Goldman obviously thinks that you can do a good job for them. You have the banking skills, so now you just have to work hard, and odds of success are fairly decent.
With that said, it's not trading, and if you want to trade, you'll regret working at Goldman. But based on the odds, Goldman is a no-brainer decision. Two years there and you have an excellent resume and you can essentially start over. If you still want to trade, best case someone takes you right out of Goldman, worst case you go to b-school for 2 years and then get back into trading at a bank or fund or whatever.
So my vote is Goldman. If you decide that you really want to trade, you at worst delay that career path by 4 years. Seems like a long time to you now, but it isn't in the end. If you go to a trading firm now, spend 2 years there, the firm goes under, their strategy dies, or you just don't make any money, etc - you'll have a much much harder time making it back to where you can start today.
Good luck.