It's basically a tradeoff. Given the same battery chemistry an 18650 carries more mah so it'll last a long time on a charge, but it's about double the length of an 18350. If you want a more compact rig and don't mind charging your batteries more often then go with the 18350, if you want longer run time go with the 18650. Using a good battery I'd say most people could probably go a couple of days on an 18650, and about a day on an 18350. YMMV though depending on your vaping habits.