KR8 seems to vape a bit hotter and is supposedly a full 3.7 volts and 510 is 3.2 volts - it's probably not the voltage as much as the typical standard resistance atty/carto ohms sold for each of them combined with their voltages. And you can't use LR (low resistance) atomizers or cartos on slim batteries without risking destroying the battery. If you want hotter hits, you need a fat battery model (Riva, Ego, etc. and then you can use both standard and LR attys/cartos)
My KR8 may just seem hotter because I use Vapor4Life cartos on it, usually the WOW cartos, and they are slightly lower resistance. By contrast, my 510 with a Boge standard resistance carto seems to make more vapor but that could just be because Boge cartos are stuffed a bit looser than V4L WOW cartos.
Most people get sick of atty+cart pretty fast and start using cartos and if they have manual batteries may also use a drip tip and drip on the atty. 510 comes with atty and has lots of blank cartos available, few decent prefilled. KR8 comes as a 2-piece, plenty of prefilled cartos available and plenty of blank. You can use a DSE901 atty on it (or other atty w/ an adapter)
I enjoy both. Downside of standard 510 is short 180mah battery life w/ standard batteries vs KR8 280mah for standard KR8 batteries. the Joye mega xl 510 batteries are 280mah but make the 510 138mm long w/ atty+cart and 128mm long w/ standard carto (I use with carto). I love the 510 mega batteries but the only PCC available for megas is the sharp-edged non-roomy Joye style so I use standard 510 batteries and a sweet roomy PCC from NoSMokeVA.com
Upside of 510 - since I use and recommend only MANUAL batteries, the 510's are great manuals because the threaded end is totally sealed metal. Super super safe for dripping on an atty or for refilled cartos (since after refilling, no matter how careful you are, occasionally a carto will drip some liquid right where it could kill or damage an auto battery). The KR8 manual still has a hole on the end - just doesn't have a drag sensor to kill in there so it's not as dangerous. But I definitely prefer a fully sealed battery end.
I couldn't decide either and that is why I own and vape both. I bet you will eventually own both so don't agonize over it.