I went with a 3kW water cooled but a lot of folks use 2.2kW with no issues. The only drawback I've read on using an air cooled spindle is if you ever need to run longer jobs at low speed the spindle doesn't cool itself properly. I've read you need at least 12k rpm to adequately cool them. Water cooled will run forever at 6k rpm with no issues.
Again, though, that's what I've read (in several places) and have no experience with an air cooled spindles other than I've seen a few. In two years I've only run our water cooled spindle at 6k rpm one or two times.
David