The E11 is very good for the money, one of the best. See how it compares here and make your choice based on what you like: http://www.head-fi.org/t/596482/the-sub-100-portable-amps-shootout-11-1-amps-compared
Rankings of the different amps follow the reviews.
The M50s will benefit some from an amp. Probably depends more on what volume you listen to them at. If you are close to max volume or like to use EQ boost at higher levels, then an amp will give you better dynamics and let you play it louder. I personally don't use an amp with my M50s with my Samsung Note 2 or Sansa Clip+ because I don't need the extra volume and the little bit of difference in SQ just isn't worth it to me.
One advantage of the E11 is the bass boost if that's what you are looking for. That can be a plus if you are a basshead
Do you ever use them with your computer? If you don't have a good soundcard with it, then an Audioquest Dragonfly
or FiiO E17
could be a nice upgrade. Both of these are DACs + headphone amp. The Dragonfly is for computer use only. But the E17 can be used as a portable headphone amp with your S3 and is generally considered comparable in SQ to the E11s headphone amp.