It depends on what you plan on doing now, or in the future. If you foresee doing casual skirmishes for the rest of your natural airsoft lifespan, then go high-caps.
If you plan to expand outwards onto scenario and milsim based play with experience, start the right way now, and source at least non-rattle can mid-caps.
For my M4 and MP5 - I originally had just all hi-caps. I regularly play the M4, so i converted over to 5x120rd mid-caps with 1 stuffed full hi-cap as emergency reserve, to lose the noise-annoyance factor. The Mp5 that I have I have used very little, so 4-220rnd high-caps is all i have right now - not worth the expense to convert over for a little used gun. It's more a back-up/lender gun anyway.
The M14 (actually, hmm, 3 so far) came with hi-caps - and am slowly replacing them, and adding to the 2 mid-caps that i've picked up so far. The mid-caps get used first, before resorting to the stuffed full (to prevent rattling) hi-caps.
I don't plan on doing any heavy mil-simming, so mid-caps are a good compromise.