I'm a firm believer in trying to cram as much power into a single rig as possible. Buying (4) 5830's is cool and all - and yes the 6990's use more power and generate more heat - but if I can get the same BTC generation power out of two cards (versus 4) - why not go for it?
For me it came down to price. Four 5830's were ~$450 USD. Two 6990's were over $1400 USD. That's a big jump. Your setup is certainly cool though.