Me personally, I'd go for the 2 20's, and try different ideas I have not done before.
Have you ever setup a dirt tank? If not, read up around here and follow someone else's tank design/layout. Never used any CFL type bulbs maybe go that route..
Think outside the box and see what cool ideas you can come up with.
Either way 2 tanks gives you more room to do different things or breed something different in each tank...