Hi Stan and welcome to ECF.
I have owned several variations of the aerotank and I've found that I tend to prefer the standard aerotank v2. The Mega seems to have a slightly muted flavor and the size creates a variance in vacuum that requires airflow/power adjustment while vaping the entire tank (in other words, I need to fiddle with it between the time the tank is mostly full and when you are almost finished with it). The aero mini has good flavor, but needs refilling more often and the airflow controller on mine is stiff and difficult to adjust. I have not tried the giant or the turbo... The aerotank v2 is much like the original aerotank and is about the same as your genitank, only with full surgical steel instead of plated brass. The difference between the original and the v2 is the airflow controller (improved in the v2) and the stock drip tip. Any of these tanks will uses the same heads as those used for your geni.
The previous suggestions for the nautilus tanks and subtanks are for different tanks. The Nautilus tanks will require different heads than used by your geni tanks (which may be important if you have a large supply of heads), but many people swear by them. They tend to also be slightly more power-hungry than the aerotanks, so, if you have a limited device, this could also be an issue.
The Subtank is a newer tank designed for sub-ohm and near-sub-ohm vaping and will require a device capable of pushing out 15-30 watts. The stock coils are also different than those used by the aerotanks--however, all but the "nano" version include the option for building your own coil like a RBA. As also mentioned, the airflow, designed for the higher watts, is considerably different as well, and may not be to your liking. One thing to note: with a little modification, you can use aerotank/genitank coils in a non-nano subtank at a higher resistance. I say "a little modification" because without any you will find that there is a gap between the chimney insulator on the head and the shaft of the tank (adding an o-ring or another insulator to the chimney can resolve this). The result is a vape that has slightly better airflow than an aerotank (flow is restricted by the head rather than the controller), a large capacity than the standard sized aero (without vacuum issues), and a slightly better flavor over all.
All of the above, of course, is subjective and is just from my personal experience. Your mileage my vary.