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The E Force and Magnuson are comparable so there's not real outstanding difference between the two. Price-wise, I think the E Force will offer a better overall deal. I think my buddy at the shop I frequent can probably hook you up with a good price on the E Force, which was a setup I considered until I decided to go the centri route. Just pm me and I'll shoot you his info!
I would take a real close look at how the two companies work their intercoolers. That's a big area where power is made or lost. The Magnuson Heartbeat supercharger was late to the C7 game, but their extra time in developing an outstanding air to water system is worth a look.
The magnuson unit is far superior in iat control. It comes w/ a varimax pump, better heat exchanger, and bricks. We did all the beta testing way back and it did outstanding in AZ. If you want some pricing let us know! We can walk you through any of the install too.