Merom (Mobile solution for C2D) is the superior to anything else in the market for audio/video encoding at the moment ..no doubt about that. However, this is a factor if you are going for at least medium to high end C2Ds.
But among the choices you mentioned, all of them are pretty evenly matched.
So for:
Power efficiency: Intel C2D T5250 (1.5Ghz)
Cost: AMD Turion 64 X2 TL-58 (1.9Ghz)