I prefer airsoft myself, and for a fairly wide array of reasons.
There is the MilSim apsect of it, in fact I tend to lean towards the extreme end as I would love to see more fields adopt a realistic capacity magazines.
You are are able to use more tactics than paintball (which tends to be a lot blind firing spray and pray IMHO); in fact if you dont employ some semblance of tactics thenyou will assuredly fail.
The weapons having such a resemblance to their real steel counter parts can really help to draw you into the different scenarios you will be playing out.
As has been stated airsoft BB's are a lot cheaper than paintballs, leaving more money for an accurate military style loadout.
And overall the atmosphere of airsoft is something I find much preferable than that of paintball.