IMO, it depends on your goals. I find that Mast prop does seem to be a little more effective, for competing. But remember, when competing, your goal is to look perfect on one day. Mast E works very well. I don't know if I get quite as dry and hard on it, but the difference is very small. The benefit of Mast E is that it works well for longer term use. If you are a non-competitor, looking to be ripped through the summer, 400mg EW of Mast E will get you looking leaner and harder, starting in a few wks and lasting though the cycle. This might be more ideal for an average guy, looking to maintain a hard grainly look. Mast prop will do the same thing, but will require a lot more injections.
I've actually enjoyed GP's Mast 200. It's 150 mast e, 50mast p, per CC. It kicked in fast and kept me looking good for 10 wks
As for BF%, the leaner you are, the more dramatic the effect of any AAS will be