The AMD chips have a higher TDP. which means they draw more power to do the same amount of work.
Also, Their clock rates are about the same but the actual performance is different.
Single threaded performance for the AMD Phenom II @ 4GHz is ~ Intel's Sandy/Ivy @ 3.2GHz
If you want Theoretical performance. Look at synthetic benchmarks that are widely available online.
The i5 2500k is probably your best bet if you can snag a great deal (Like the recent $99 MC deal).