Greetings; These devices are really apples & oranges. If you're thinking about the Mega because you want more features (e.g. such as Var Voltage) than what you have now, keep in mind that the Innokin device pretty much has all the features that it is possible to get -- VV, VW, Ohm Reader (I think), etc. It's a full-fledged Advanced Personal Vaping device.
I personally went through a slow, frustrating and costly progression of beginner eGo batteries before I finally bought an APV that gave me the kind of vaping experience that I really enjoy -- my advice, FWIW, is that if you're already committed to vaping, and not just experimenting, don't waste time with the Mega.
The drawback with the Innokin VV, as has been pointed out, is battery time. But if you're often around computers during the day, it's easy to recharge on the fly, and the Innokin VV has pass-through capability (kind of) -- IOW you can vape while the device is recharging. If you're not around computers a lot, maybe it would make sense to be patient until you can afford to spend a bit more, and get something like the Innokin MVP, which has a 2600 mAh battery that should last for a day or two between recharges (if not longer).
Good luck!
Regards
Glenn