Depends on the client. If you have clients that are database intensive and require frequent memory swapping - then the i7 is recommended. If they're static sites, then the i5 might get away with it.
Also, performance changes depending on the i7 chip you get. An i7-2600 (3.4ghz) will perform much differently than an i7-3930K (3.2ghz). While the speed appears slower, the i7-3930K chip handles much better performance.
The only thing I can advise you is to do your research, and compare to the Xeon while you're at it. Pay no attention to reports about overclocking the CPU - you don't want to do that with a web hosting server!