I recommend the king arms m4, Very cheap if you look around, I found a good price for it (can't give retailers till age verified), Try searching the gun you want of Google, don't rush buying that is what I learned.
+1 On the king arms, Don't buy G&G if you know if it is to expensive, you can get the same;better quality in other company especially King arms with there line of m4s
For the mp5 thing I would recommend getting the m4 if your playing some type of big indoor game or outside, if you play like at Sgt. Splatters I would recommend the m4.
The m4 barrel is much longer, If you get a stock barrel of a m4 it will have better accuracy than the mp5 barrel, and it depends what type of company you are choosing, (sometimes there are different sizes of barrels in the different guns, for example a stock king arms has a 6.08 barrel and a Kwa has around 6.05 from what I hear.
Wait for a month or 2 and search some sites a few times a week, you can see what time gives you.
And please read the laws of Airsoft, and don't buy guns outside of Canada it will get seized.