All else being equal, I'd recommend a M4 over a MP5.
M4s are IMO, more newbie friendly. First of all, almost all M4s have an adjustable stock, which allows for better adaptation of new player to new gun. Comparatively fewer MP5s have adjustable stock.
Second, M4s are easier to accessorize, because of the sheer availability in variety and quantity of grips, stocks, etc. This makes the M4 a little more "forgiving" as a first purchase, in the sense that if something isn't right ergonomically and/or functionally, it can probably be readily replaced.
Third, pouches for M4/16 magazines are more common with more varieties.
Of course, the ease of accessorizing sometime turns into a curse, when people just can't stop doing it...
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."