I didnt vote, i haven't used both.
i have a 24oz co2 tank as of right now, attached to a remote line.
it's an aluminum tank (as opposed to steel commonly used in CO2 tanks) so it's a bit lighter.
my velocity (NO REGULATORS) honestly stays within 10fps, usually less, which i'm happy with.
i haven't tried colder weather yet, but i have a scenario coming up on october 12th, so we'll see how that goes.
i plan to get a nice huge HPA tank and use it, but sinc ei have a response trigger i'm probably going to still use CO2 some of the time.

__________________
Proud Canadian Baller
Kit: A5, camo job, polished internals, 14" Smart Parts Progressive barrel. Borrowed Opsgear G36 stock. 24oz CO2 w/ remote and slide check. Neat homemade sling. MORE TO COME
Vents Goggles and Multicam