Jen, you may have noticed that the answers you are getting bounce first one way and then another. KR808D-1s have a lot of fans, and 510/eGo ecigs have a lot of fans. There simply is no absolute best, only what you personally like the most. I tried DR808D-1s and for me they did not produce as much vapor as a 510. But others absolutely love 'em, and don't like a 510 at all. Some love cartos (I don't, but prefer atty/carts by far). And keep in mind that besides the eGo, there are 510 xl batts with 280 mah if you prefer a 510 with longer battery life, but small form factor.
I wish I could say that you will make the right purchase if you....(fill in the blank). But honestly, you may have to try both, and even others besides, before you find what you really like.