That's a real hard one to answer as a number of things will affect how long your computer will last and also the fact that the Apple is not 'user-serviceable' would be contributing factors to how much it may cost in the unlikely event that it will breakdown in the foreseeable future.
I too, like you, have an iMac that's just over a year old, but also got PC's which I'm quite comfortable tinkering with.
If you're savvy with the insides of a PC then it may not pose too much of a problem for you should it break down. Whereas the Mac may cost you a bit of dosh if it's innards were to be exposed by a 'professional'.
Ultimately, you have to ask what's driving your need for the Apple over the PC or vice versa. For me, it's the software and the decent screen. A screen is easy, but OSX and it's various installed Apps I have are not available on a PC so I'm staying with a Mac even if it does take a tumble.
I guess the main things that could affect how long your Mac, or PC for that matter will last are:
Power (surges and spikes)
Heat
Environmental factors such as dust etc etc
I would put heat as highest ranking nemesis on your list.
Should anything fail, the most likely thing to go will be the Hard Disk - being mechanical and rather hot running. These are not expensive to replace as they used to be and frankly, by the time your Mac needs a new one, you will probably get something with a lot bigger capacity for very little money. Just shop around at the time and you'll see what I mean. I'd never buy a Disc off Apple without first checking the price of other hard disks elsewhere. Obviously, a motherboard replacement is a completely different matter altogether.
Apologies if this hasn't answered your question, but thought I'd pop my 2c in.
Cheers
R