Would you spend the $100 on the upgrade in drive size? $100 is pretty cheap for 360GB. On the other hand, most 3d party drives there's about a $30 difference between the 500 or 750 and 1TB.
I upgraded to the 1TB as I thought about the hassle a couple of years down the line.
I remember building a PC for a friend years back and telling him if he filled a 10GB hard drive, I would replace it for free. Within 6 months he was making videos at 2GB a time and soon ran out.
Best to get it now rather than struggle to upgrade later down the line.
That's what I'm planning to do when I buy my iMac; paying a little extra for the 1TB model. I figure it's a better value for the money and with the increased space, my iMac will last a little bit longer (5-6 years hopefully). But, I also download alot of music and movies and will probably play games on it, so the extra space will also come in very handy.