Running in dual channel is "faster" in benches and testing, but 3GB will give you more headroom in games and apps. I hear StarCraft II likes to run with 3GB or more of RAM.
I'd say 3GB. Dual-Channel is a bit faster, but you'd gain more performance with space rather than speed.
Nowdays, some boards will run Dual-channel with two sticks of different capacity, but they have a different mode.