The 2700m is based off a 9700m GTS part. The 3600m is based off an 8800m GTS part. The 2700m has a 30Mhz clockspeed advantage (530Mhz vs. 500Mhz on the 3600m). Both have an 800Mhz memory clock, 256 bit memory interface, and 512MB of memory.
In all honesty with only a 30Mhz clockspeed advantage, they're going to be right near identical to each other. The difference is that the 2700m is built off a cooler running and lower power 65nm architecture compared to the 80nm architecture of the 3600m.
Unless the cost difference is high, go with the 2700m.