Another factor that might be relevant: The typical Kr8 cartomizer is prefilled with e-liquid. The typical 510 carto is not. With Kr8 cartos you're paying for the e-liquid ... and, if it is good e-liquid, it's worth it.
When you say that there are not "that many options for" 510 cartomizers, if you mean
prefilled 510 cartos, I agree with you. If you mean 510 cartomizers
in general (including
unfilled), I
very much agree with Hoosier (although I'd like to hear more about
triple-coils didn't know about those).
If you have a regular 510 battery (rather than a 650+ mAh fat batt like the eGo), you probably should stick with the regular resistance cartomizers. "LR" (i.e., low-resistance, typically 2.0 ohms or less) is intended for 450+ mAh batts. A regular 510 batt is only 180 mAh (the XL/Mega 510 batt is 280 mAh).
LiteCigUSA is an inexpensive source for Joye, Boge, and SLB 510 cartos. Hoosier has told you about
IkenVape.
MadVapes carries Bauway (and others).
Otherwise, go with Hoosier's recommendations ... and add Smok Tech Mega Dual Coils to the list (e.g., $9/five-pack from
LiteCigUSA).
I do
not recommend using your Kr8 cartomizers on 510 batteries. They are designed for the 3.6-3.7V Kr8 batteries and their performance probably would disappoint on the 3.2V 510 battery. You can try it out cheaply enough, with a $1 510-to-808/910 adapter from
Good Prophets.
FYI: "Cart" is used as the abbreviation for cartridge (those things that ineffectively try to deliver e-liquid to atomizers). "Carto" is the typical abbreviation for cartomizer. Here's one reason why this is of more than just academic interest: I know about cartos, but I abandoned carts long ago. Almost didn't bother looking at this thread, since I didn't think I could be of much help.