If you haev a class that does really good AoE damage (Like Ret paladins?) then the lower drop rate of the phase mobs is countered by their simplicity.

The run time spent running in and out of the instance is not efficient, so only killing the first 3 mobs will not be as fast, and I've done it both ways.

The only way the two 5 mob pulls would be any harder is if you only have 3 CCs available, if you have 4, you should just be able to have each of the CCers set focus on a mob, and cast their CC on focus target. With that, recasting CC in combat is simple.