For me it always comes downt to the OS, because thats what I interact with. The UI needs to work with me and my work flow, if it doesnt, then I am not interested, I dont care what the specs are of the machine. I have a MacBook Pro i5 for work, a 27 inch iMac i5 at home and a Dell I did a bit of modding to. i7 chip, Nvidia 560 ti, new PSU 700 w running Windows 7 home premium. The PC is a fast machine, really moves, but its Windows at the end of the day. Now Windows 7 is a good OS, and it was about time for Microsoft to come out with a good OS, but I still prefer OS X by leaps and bounds.
I use what works for me, I am loyal to no brand and never will be. I do have more invested though in the Apple eco system with my iPhone 4, iPad, multiple macs, plus iTunes and the App store.
I too have a very Mac based eco system around the house. Iphone (wife and I), MBP 13" for email and apple TV. That along with OSX are the only reasons I am even considering the iMAc. How does your PC tower integrate with your macs in your home network and file sharing etc? That is the stuff I am not too familiar with. What it comes down to is that I have to decide what I want out of this computer. The intention is to set it up in my office and maybe do some odd freelance work (Avid editing mostly as I hate FCP but have it on my MBP wich runs slow) but have something more powerful so I can actually do work. Anything on my MBP is now painful as the C2Duo just doesn't cut it.
Either the imac or a homebuilt rig would do most of the light Avid work but where there is difference is that the homebuilt could be expanded on and I can add things as jobs might require. That is an appeal to me. When working within programs the OS doesn't matter much to me but it is the other stuff where OSX is my preferred choice. I honestly don't have a lot of experience with Windows 7 other than my workstation at work.
Both appeal to me in different ways. It is hard to pay the same money for much less with the iMac. If they only had kept the ability to swap out drives or even add an SSD into the provided craddle without having to go through built to order I would be all over the iMac. It makes no sense and leaves a bad taste. I've already upgraded my hd in my MBP and you can still in current models. Having to show up at the apple store and hope its within warranty for anything wrong just scares me not to mention the possibility of being down for weeks if by chance I am in the middle of a project at home. I know I wouldn't buy a bike that could only be serviced at the most expensive dealership in town. is OSX worth it? that is what I will have to come to terms with.