Go to the Ask A Veteran section and you will see a thread that they answered for me about the difference between the 901 and the 510.
I use a 901, I have tried a 510 and it seemed pretty similar. I have stayed with it because it has worked for me thus far, but if I were just buying one today, I would start with a 510, because your first kit will have 2 atties and 2 batteries and a charger. The atties will more than likely die on you first, leaving you the batteries and charger. If this has worked for you, you can stay with the 510 and order another kit AND some extra atties. Even if you vape heavily, this set up will last you 2-3 months. The cost, if you shop around will be under $100 (minus the cost of juice). The 510 is just becoming the basic standard and will give you more choices than the 901. IMHO