Ima long time 901 vaper and i have tried the 510...
The 901 gives mouthfulls of vapor with incredible flavor, and is not so strong that it burns your throat or lungs like the 510 does. 901 doesnt leak like the 510 does and honestly, the 510 is a piece of junk imo...
Downside with the 901 is its performance signifigantly drops after about 2 weeks.
Just ordered myself a 902 cuz im hearing great things about it. love the 901, cant stand how harsh the 510 is. So maybe this 902 will last a little longer before i gotta replace the atty and if it does, ill be set.