I voted for the iMac, since you're considering an all-in-one. Much like a laptop, an all-in-one is hardly upgradable, so the "can't OC" and "can't change GPU" arguments against the Mac aren't very applicable.
Plus, I know you like Macs, so you have no problem with the OS.
Also, there's the advantage of not having to deal with adware and the like.
If not, then the Sony all-in-one series are also very nice. I believe I heard about Sony giving the option to NOT pre-install adware?
Personally, if I were looking to save space, I'd just get a SFF case (like the one in my sig).