i7: 8MB cache, 4 cores, 8 threads, 2.5ghz base clock, 3.7ghz turbo
i5: 6MB cache, 4 cores, 4 threads, 3.1ghz base clock, 3.5ghz turbo
I imagine it depends on what exactly the turbo bins are on the i7, how long it spends in turbo, and how threaded your load is.
I'd take the i7, personally.