Ya want something with a little battery life AND cig sized, go with a KR808D-1. and use their mid sized 380mah.
You should easily get 4 hours out of it, or the complete usage of a carto. At that time (carto used up) ya gotta change up anyways, so no biggie.
(likewise, those with these cigar sized units gotta at some point change carto's so they are dead for a short time as well)
It takes about 20 seconds to change off a carto, but 2 seconds to grab a spare battery/carto setup, and you are on your way.
I keep a minimum of 3 batteries (but more often 6) already set up with a variety of carto's. And at night, I charge and tap up carto's. The whole 6 setups take less space than a pack of cigarettes.
It comes down to what you want to use.
Cig sized/shaped , or some monster looking thing.
I like cig size, and have for the last year.
Also the 510's are very good but the battery life is a little less.