sst 50 should product a more even beam due to the chips being very tightly spaced. I've only used mc-e's but they tend to have a dim spot in the center of the beam if you try to use them as a spot. I believe p7's have this problem as well.
I don't really know which is the most efficient, but with the amperage requirement of an sst50 you can run into thermal issues.
Also I like the wide variety of tints available in mc-e all the way from warm to cold white as well as multicolor.
