Just speaking in broad generalities, reserve on the 757 is probably better, but if you're thinking about sticking around long enough to be a lineholder, there are plusses an minuses to each. Expect 12-15 days off as a bottom lineholder in either fleet. 757 out of EWR is usually commutable on both ends, 737 usually isn't. However, the 757 flying out of EWR is hard on the body with 5-6 crossings a month.