Im not a pro. These are my observations, so far.
At 3.7v
801 - Cool vapor, average flavor, low throat hit. Average vapor production.
901 - Warmer vapor, good flavor, average throat hit. Good vapor produciton.
510 - Warm vapor, lots of flavor, big throat hit. Heavy vapor production.
As you raise the voltage, the vapor gets warmer. Eventually the 501 will get to the point of being to hot and will ruin the flavor to me. The same can be said for the 901, though it takes more voltage to get there. At 5v+ I prefer the 801, then the 901.
In my case I use mostly 510's, and just lower the voltage. Some people do not have that option, or like a device that doesn't have that option, and changing the atty is a reasonable solution in such a case. I have also been in the sad situation of not having an atty I like in stock, and having to find something else to try, the above has served me pretty well so far.