Personally, I prefer to find a good choke point, dig in and play the waiting game until I can get the army set up the way I like it (depending on what sort of tech I have in my use this could be a mid/late-game steamroll or small tactical strikes slowly chipping away the enemy) all the while conducting precision harassment as possible.
However this is what I do in single-player, do not really care for on-line as most people there are pretty much dedicated players that will stomp you fast or start bitchin on something you did or did not do.
However, having witnessed quite a few on-line games (C&C3 KW), I would say that quality along side player skill will out-perform simple quantity.
For example you can with careful management get a stealthed apc into the enemy base carrying a commando that is capable levelling structures with one explosive charge and while they go about stomping the battlefield with their many units, you can pretty much remove them from the game before they figure out as to what the fuck has just happened. This is even more profound what they amass one specific unit type and then realise the magnitude of their stupidity, as quite a few games feature damage specialisation, they have a shitload of anti-armor but anti-infantry, so even if they notice their misfortune they do not have the resources to do anything about it.
People have ragequited because of that, fucking hilarious, even more so because it was done by the person whose base they had nearly decimated.