That's just marketing bullshit and spandex boy hype. When my main site was The Mall (too soon, RIP), I would regularly play with full length M4s, a SCAR-L, an M249 (no, really - it was hilarious). I've seen people playing with M14s (granted, the SOCOM short barrel ones but still).
Unless you're playing in a broom cupboard, 100mm vs 200mm is completely moot. Get the long one.