Personally, I would go with the iMac.
With a custom build you'd have to run Windows 7.
Which from my experience isn't anywhere as nice as Snow Leopard (I've used it since the first beta came out, up until now with the final version.)
And also another thing, you wouldn't have any kind of insurance for it, and PCs tend to break a lot sometimes, once again, from experience with them.
So, if I were you, I would definitely go with the iMac, because the custom build might look cheaper now, but when one part breaks, you'll have to fully pay for it, which will end up being more expensive than the iMac to begin with.
And if the price is not something you really care about, you can never go wrong with a iMac because of one simple reason, OS X.
And the iMac is good enough to run pretty much ANYTHING you want it to run for today, and at least a few more years.
The custom built does look more powerful.. but then again, if you think about it, why would you ever need to use 4TB? It just seems kinda silly, and I doubt that anyone really needs that much space, unless you're a serious movie editor who works for a major movie company and space is a must have.
Pretty much what it boils down to is Windows 7 or OS X?
Which one do you want to use more?