I think it's just a matter of preference. Between Mobos and Zerkers, it's about whether you want your presence spread out, or if you want it to be stronger where you already are.
Some things that may influence this decision could be how well your lanes are fairing. If they're all having trouble, then you might want to spread yourself out a bit more, and get the Mobos.
If only 1 lane is having trouble, then Zerkers might enable your ganks and counter-ganks to even that lane back out.
If all your lanes are stomping or even, then your goal should be controlling objectives. Since all the non-tower objectives are time-based, then you don't particularly have to be in a hurry to get to them if you're organized, so you could consider the Zerkers to make killing them a bit easier.
It could also have to do with how often you use Deceive to get around. The distance you can get with Deceive has nothing to do with your MS. Also, if you're in combat a lot (farming or chain-ganking), then your Mobos aren't going to be active as often, but if you recall frequently and/or find yourself moving cross-map a lot, then they'll be active a lot more often.
So yeah, preference. What do you want to boost? Combat, or between Combat?