I don't know if Zong has launched this or not, but I read Zong don't support 256QAM (Jazz does), hence the lower COMPARATIVE speeds (at least on my phone).
I have noticed that even in piss-poor conditions and zero aggregation, sometimes Jazz will still be significantly faster. I just did this test right now, at my home:
Zong:
Mobilink, on a single carrier, gave me around 7 Mbps. When I ran it again, it said dual carrier kicked in and showed this:
Since the forum isn't letting me attach more than 3 files, the dual carrier stats are:
1800Mhz:
RSRP: -111dB
RSRQ: -14
900Mhz:
RSRP: -111dB
RSRQ: -7
In the end, both are comparable at different spots - it depends on how clean your signal is. I've personally hit 100 Mbps on Jazz (averages between 20 and 60), but the highest I've seen on Zong is 60Mbps (averaging between 20 and 40 in my limited testing). My sister's place doesn't have strong Jazz signals (100dBm max) but she gets 70Mbps inside her house. Zong with 90dBm doesn't go beyond 20Mbps, with aggregation.
You can't go wrong with both for general usage - except the likelihood of receiving a strong signal is higher with Zong, but the likelihood of getting faster speeds even in pathetic signal areas with Jazz is much higher - and I suspect this is owing to 256QAM (without technical knowledge). Zong probably went the aggregation route to achieve similar performances, but aggregation can do only so much alone.
I just tried again (before posting) to check consistency and got similar results - Jazz on 35Mbps in pathetic conditions (with aggregation) while Zong, in less pathetic conditions, not going beyond 3Mbps. And I see this change happen as I step inside my house. Outside my house, I've seen Mobilink give me 10Mbps in average signal conditions while Zong fluctuates between 3Mbps and 70Mbps.
Yesterday evening, on the first storey in a house in F-8/4, Zong (with carrier aggregation, both around -98dBm RSRQ) gave me around 15-20Mbps. Jazz, with similar aggregation and signal strength of -94dBm or so, gave me around -50Mbps.
So it's still apples to oranges, but for the most part, Zong's speeds have tremendously improved, and for data, you can't go wrong with both as they'll give you similar experiences in the majority of places (but in weaker signal areas, Mobilink, 90% of the time, will still beat Zong)
Oh, and I've been testing for a few weeks now so this isn't a fleeting test - these are my observations.
If I were a new customer - I just might opt for Zong cause they have aggregation, stable fast speeds, and their signals are in many places (except where they're not). But on the other hand, where I can get a dot on Jazz 4G, the speeds will be decent enough so it's a moot. For voice, Mobilink falls back to 2G so much it's become a joke (haven't used Zong for calling so I can only refer to the PTA QOS report - which said they don't fall back to 2G much).