All I can say is that if your preferring to use factory stock heads in any tank whether they are single, dual or vertical your in for a rough ride since they are mass produced by the kajillions with no concern of the end result. I've been rebuilding and using my protanks(1&2) for eons now without any issues what so ever but thats me. Single bottom coil attys that are rebuildable to get you started then from there once learned how to rebuild something easy like a protank head then advance to more sophisticated builds like in a dripper or kayfun will just fall into line.
Good luck with what tank you could want but I get consistent hit after hit with my rebuilt protanks tank after tank and can vape the juice right down to the last drop.
Totally agree with AlterUrEgo statement about stock coils, regardless of tank stock coils at times can leave a lot to be desired.
My preference is the Kanger EMOW Mega, 19mm diamater looks great on my iStick and I rebuild the enclosed dual coils to a vertical coil wicked with japanese cotton. Suits my needs and can't think of any other reason to use anything else.