IMO the 510s produce more vapor... but it seems to be a very fine, light vapor
The 901s make a lot of vapor too even at 3.7v, but it seems thicker and denser than the 510. I get a better
throat hit off a 901.
Flavor-wise I think the 901 might give a little more flavor, but not too much
If I had to choose between the two I'd probably take both. They both have their place. Although, it seems like a lot of people prefer the 510 over the 901 especially at 5v.
If your going to use a 510 atty, don't use it with the stock 510 battery. The
batteries suck and they only put out like 3 or 3.1 volts. Get a 510/901 adapter and use it on a KR808 batt which actually puts out a full 3.7v on a consistent basis.