What is this gradient issue you speak of? Are you referring to viewing angles? If so then I have to agree that they aren't that great and it is best to view it head-on. Although this is pretty much a none issue for me. EDIT: Okay I Googled it and I have noticed no problems with my iMac.
The MacBook is a good machine, however as a desktop replacement you would want to use a monitor rather than the 13.3" display, which would then make me question whether your Mac Mini would work just as well unless you need that portability. I have a few niggles with my MacBook as well. The fans get quite loud, even just what I would call small tasks like watching YouTube videos (I don't have the same problem with my iMac) plus after a few months you will notice a little crack appearing to the right side of the lid causes by the magnetic latch. However, it is a great laptop. It is fast, a nice size for what I use it for, the magsafe is awesome and I love the keyboard.
I don't think you will go wrong with any of them. However, for your budget and requirements I am inclined to go with the white iMac. I can't really justify the added cost of the new aluminum iMac if you are happy to get a second hand white iMac.