You can't use the GHz to compare across makers and even if it was the same maker CPU you have to keep within the same models to guess what the payoff would be.
Example. A dual core Centrino (yes I know, dated) at 1.3GHz could best a 2.4GHz P4 HT. This was a good example that you can't compare by clock rates.
What to do? Go find benchmarks on those 2 CPUs. Or better yet try each machine in the stores.
Not an offer to find those benchmarks but a parting comment. Those are very high end machines and as such may be LOUD. Have you heard them running?
Home computer, general internet, email, some Excel, Word, basic photo editing, light video streaming, NO GAMING. Will I notice a real world difference between an Intel 2.5 ghz core 2 quad Q8300 running 6 mb DDR2 and an AMD 3.0 ghz quad Phenom II X4-945 running 8 mb DDR2, both Windows 7 64bit? The AMD computer is $100 more and large, the intel is small form factor, which I like. Thanks for your input!