I grow tired of my G36C, and so when I stopped by AEX and saw both the M15 and Mp5 I was looking at I thought to myself "why not?". But I live on a barely non-exsistant National Guard salary, so I have to choose carefully.
same gear box almost exaclty if not exaclty. CAs are CA good gun not great but not bad, I like them. Sometimes they have issues but they are getting better. Which model real comes down to what style your playing.
honestly, both of those guns are extremely common now a days, if you want to look like everyone else go right ahead and pick p either. in my experience the mp5 served me better, never pooped out on me, and shot like an arrow. personaly i like mp5's more and they are both equally upgradeable and will shoot fine, but you have to look at what your style of play is. what will suit you better...???
I have a CA M15, and it has serves me well. I fell on top of it and then a friend of mine fell on top of me on top of my rifle and it still shoots nice and straight.
Shame the Mp5 is so saturated, I always liked them. But I use an M4 in training, so I guess using one for airsoft I can just skip the whole "getting aclimated" portion of buying a new gun.