I'm afraid I can't offer any advice based on first-hand benchmarks with Hauptwerk, but from benchmarks/comparisons I've read of the i7s, e.g.:
http://www.tomshardware.co.uk/intel-core-i5,review-31674.html
... I would guess that the older 3-channel i7 (920) might give better performance with Hauptwerk. Being able to support 24 GB is also definitely a plus.