Being of little experience and having spent more money than I should have experimenting with the next best e-cig in somebody's opinion, I would recommend, like others previously that you look at the information in the "Sticky" threads first and foremost. I have read through tons of these threads and it is just overwhelming without having any practical experience with one of these devices to use as a reference point.
I have tried 4 different models in just over a month since starting into this and can say that with a little patience and experience I was able to get similar performance out of all of them, once I figured out how to use each one. While optimal performance may be similar, how long that can be sustained can differ drastically, mainly due to battery capacity and cartridge/cartomizer size (i.e. fluid capacity). Go through the sticky threads to get good generic information on the models, basic operation, etc. and then pick one that you think fits your needs and then play with it for awhile before jumping into the next best thing.