Closed loop will definitely be the cheapest up-front purchase and installation, but on-going electrical usage will burn a hole in your wallet to run such powerful pumps. Also, take into consideration that those external pumps create a lot of heat, and you may need to run a chiller more often than necessary. I know it's a peninsula tank and you don't want powerheads coming off all the panels. MP60's are also a lot of flow in one spot when dialed all the way up. You may want to use mroe MP40's on the overflow box side alternating pulses throughout the tank at different depths. Many people say this can't be done in such a long tank but I've seen it work very well.
For instance...
http://www.youtube.com/watch?v=zh6Cz_H8Uhg
Take note that this is a 10 foot long and 5 foot wide tank that is 3 feet high (1200 gallons). Granted, that's $3,200 in Vortech pumps, but at that size of a tank the electrical savings from closed loops is saved pretty fast. It appears the reef is thriving. I notice there is no obstruction in the path of the Vortech pumps the entire length of the tank on both long sides of the penisnsula. I'm assuming you won't be piling rock near the wall either since you'll be viewing it from 3 sides as well. I remembered this tank, and thought I'd forward it along. You may be able to get away with 4 MP 60's on the overflow side, but I'm pretty sure that it will kick up some serious sand storms drawn into the propellers and strewn across the reef.