The Dual 2.7 is a LOT of power!
I was once (well like 2 weeks ago) a speed freak. I always wanted a faster and more powerful rig. I was planning on building a Athlon 64 X2 ( the new dual core chips) with water cooling to be silent, cool, and powerful in my bedroom. I had been flirting with the idea of Mac for a while...but the one I could afford was the iMac...a paltry 2.0 GHz? My current Athlon rig was OC'd to a 2100+ (1753MHz). But, I bit the bullet and bought it.
And you know what? It may not be the fastest, but I don't care anymore. Stuff is just so pleasant to use, it just works. A dual 2.7 would blaze, but I really don't care waiting on the Mac. t seems to multitask very well compared to Windows XP, so I can just let one task go in the background and do something else without a problem.
The iMac just looks cool, is silent, and just works. Now, if you want the dualie, go ahead, it is very sweet. But I have found that in the Mac world, not only is the MHz Myth busted (a 2.0 GHz G5 feels quicker than a similar P4), but also, speed really doesn't matter. It seems that RAM is just about as important, if not more, in determining your "speedyness"
Both systems have thier advantages, but I don't think you can really go wrong with either one you choose. My next Mac may well be a dual PowerMac, but we'll see when the time has come. Good luck!