What do i need/want in a fan? I would rather buy one good one than have to upgrade later.
Are you referring to an exhaust fan, or one for circulating air in the tent, or both?
For exhaust, I really like an inline duct fan with a speed controller, blowing out the top. This, paired with a filtered intake on the bottom, mean lots of fresh air.
For circulation and eliminating dead spots, those cheap clip-on 6" fans are great. They can be clamped to tent poles, or some kind of brace can be easily rigged if you need to mount it in a different location. I like the cheap ones because in my experience, they tend to gum up eventually anyway, so dropping $10 on a new one every year or two is insignificant.