I started with the 510. I loved it. Not as flavorfull as I would have liked, and maybe not exactaly clouds of vapor, but enough. I never bought another pack of cigs.
Then I picked these up:
PV Adapters
I got a 510-901/808 and a 510-801. I found that the 801 produced more vapor. Now I do have to mention that I don't have any carts for it so I have to drip. Obviously with so much vapor there was significant throat hit, but still not the flavor I wanted, even dripping.
Then I tried the 901 and fell in love. Decent ammount of vapor, but the flavor is amazing. I've made four simple mods, (licorice tin mod, flashlight mod and a pair of nicostix.) Three of the four mods are 901, one is 510.
I still don't have actual 901 batts, I use my 510 batts or pass or a mod.
901's are easy to mod because you can buy a pretty good battery connector from radio shack, (M or N type DC jack).
Hope that helps some.
Pick up the adapter, a 901 atty and a pack of carts, you won't regret it and with the adaptes it won't break you to try.