Man, I just popped in my Zong SIM.
Good thing: I get 4G almost everywhere - weak or strong.
Bad thing: speeds are dismal in several places. I think Mobilink has fewer choked towers (they have really poor speeds in some places). They range from 0.2 Mbps to 20 Mbps (i.e.a HUGE variation), and if you're lucky, in places, you'll go higher.
Mobilink: minimum speed is maybe 1.5 Mbps or so, but even in weak signal areas, if their backhaul isn't choked, you can get something decent.
Zong is probably milking whatever they can and not fixing their backhaul. They have the signals, it seems, but when they're not able to deliver, then that's an issue.
Mobilink just won't bother patching up the coverage holes they have (and there are quite a few).
So... tough call on which is better - but if data USABILITY is your thing, then Mobilink is still far ahead (albeit with crap 2G).