Which is better: ATI 5450 DDR3 2GB memory -vs- ATI 5550 DDR 1GB?

Generally speaking, the more memory the better. DDR3 is also better than DDR, and after looking at the specs for both cards I don't see anything to recommend the 5550 over the 5450. The 5550 just looks to be a lower end version of the 5450. Go with the 5450.