I ordered the cartos they carried (and recommended) at IBTanked along with my new carto tank (Smok brand). When those ran out, I ordered a bunch of Boge 510 LR Stainless (Laser Punched - 1 hole) from Sure Vapes. (The price goes down when you order 5 packs at a time.) Don't see a huge difference between the two brands.
Standard resistance is fine - low res works well too. For reasons I won't go into here, I don't recommend dual coil. Machine punched or slotted seem to be a bit better, but if you want to save a buck, you can always punch your own.
As to what length to get, there's something to be said for the look of a carto that exceeds the length of the tank, but only on certain tanks.
For example, it looks really good on this:
But not so much on an IBTanked.
I'd just get the size that matches your tank and be done with it.