I'm liking the concept of smaller. Like 3 diff strains in one 20-25gal pot on a lazy-susan (tiered of course =D). A smallish tent with 1 LED and 4in exhaust. High Brix, top once, FIM once, LST, flip - let her go.
BAM!
From what I've read, go for the 6" fan and get a good regulator. They run longer and quieter than a 4" when you turn them down and you'll have a good 6" fan and regulator when you do the inevitable upgrade. Just a thought.
Other than that - BAM!