Well, he's not asking about
tanks but, there's really no difference between the Jaunty Jet, the eGo-T, the Inferno-T and the Tornado-T.
As far as the mega t-rex goes, that depends on how much you mind waiting. You do get a lot more battery life but, the regular Rivas do pretty well in that department. If you get one at Liberty Flights now, you can't get the carto deluxe which wasn't a lot more. If you do decide you can't wait, I'd get the 1100 MAH for the $3 difference.
However, you might check out Litecig. Their Riva kits aren't as cheap as Liberty Flights when they have them but, still cheaper than that Mega T-Rex.