Hiya again Cancer
Theres a huge substantial difference between the 801-901
in general size, cart life, battery life, atomizer performance, and teqniqnes.
The 901 as you may know, has a feel more like a cigarette, quick, sharp pulls gains the satisfaction, where as the 801 is more geared torwards the long, slow, deep pulls is needed to be fulfilled. the 801's body in general is a few shades bigger than a standard pen. Both are equally fine to drip with, keep in mind when dripping, you'll need to preform more routine maintenance than without.
The cart life is significant, where in the 801 will outlast the 901 three fold. Same princaple with the battery life. That said, the pic below is of a 901, 801, and 510 Judging by all the aspects of both, the 510 will be directly in the middle of the two. Everyones preference will vary with styles but its ultimatley up to your preference.
As always my friend,Wishing you all the luck -Iken