One thing I'd like to add in reference to 901 vs. 510
threads, is that unless I am reading it wrong, Imeo is just asking for a
thread preference. In a custom atomizer you can't really make the call based on flavor/etc... because the threads are not what give the flavor. A 901 gives better flavor than a 510 because of the atty design, not because of its threads.
Given that I would say 510 is better, because it is more universal.
Of course like I said I may be reading it completely wrong and misunderstood...