This is coming from someone with experience in quite a few operating systems. First and foremost. If you don't feel the urge to try linux as a major project and learning experience, don't. Linux is extremely easy to do things already set up, but the second you hit something you have to configure yourself, you'll spend hours googling solutions and typing cryptic commands into the terminal.
Apple does make a good system, and I do like the OS (more so on laptops than desktops, because the track-pad integration is great), but if upgradability is a priority, run far far away. You will not be able to upgrade any component with the possible exceptions of hard drive and RAM on an iMac.
My recommendation is that you build a computer. Find someone who knows what they're doing, and have them help you find components and put it all together. It's a valuable skill that will get you highly flexible and much cheaper computers for years.