Standard size 510, 4081 and 1st Generation 808 all have about the same 180mAh.
2nd Generation 808s are 220 mAh.
There are now all kind "mega" or "xl", or "long" versions out there for each kind, the mAh rateing can get up to 350.
(I am just talking about the "slims")
Right now, the thing that seperates 808s, is the Horizontel coil cartos out there.
The most common horizontal coil for the 510 is the Kranger at a 2.0 ohm. 2.0 ohm is intended to be used on a 400mAh battery.
Most of the accesories with 510 threading are for the big battery and mod crouds, things with "uncompatable" ohms, or would look really silly on a slim battery (picture a MAP
tank on that smoke stick).
Personally, I'd consider geting both adapters. Right now, I'm useing a 510 boge on a 4081. Makes me wish someone sold decent blank cartos for the 4081, or someone made a 510 auto that was worth a darn. (to be honest, I was more placed into the 4081s by the local availability of the nJoy but I never heard anything good about 510 autos to make me switch).
The 808s have alot of options, esspecially in the prefilled cartos. v4l, Smokeless image, and at least 20 others I could name.
If you want to try the prefilled, 808 adapter is what you want.