Get a Mac. Or get dedicated hardware, like a mixer with an SD slot and basic recording. The Mac is the one and only general purpose system that is built for what you want to do. Not just at the application software level or user interface level, but in the OS subsystems and developer API's and hardware. Basically, I'm telling you to buy a computer with CoreAudio and CoreMIDI in it — you can't bolt those onto another system. A Mac comes out of the box doing what you want to do, and you can then optionally replace GarageBand with Logic (or Pro Tools or Performer or Ableton Live) and optionally replace the built-in (24/96) hardware with accessory hardware via USB, FireWire, or Thunderbolt. Even where you plan to run something like Ableton Live which is on both Mac and Windows, you will have a dramatically better experience on the Mac because you can do things like run AudioUnits plug-ins instead of VST to improve stability, and you can combine multiple audio interfaces into one, and you get reliable hardware connections through CoreAudio out-of-the-box.
Also, consider getting an iPad mini as a remote control. It runs MIDI mixer controls, pianos, drums, guitars — most of which can output MIDI over Wi-Fi to your Mac. Logic X has its own iPad app that runs it from the iPad and adds transport controls and mixer faders and so on and just works. The iPad will cost you $299, yet it will replace thousands of dollars of MIDI gear. And when you only have the iPad by itself, it becomes a complete pocket studio if you add an Apogee MiC or Jam. A7-based iPads can do 32 stereo tracks in GarageBand (earlier iPads do 8 stereo tracks.)
A big thing is that the Mac is where the user community is that is also doing what you want to do. So when you struggle with something, there will be someone who can help you dig out of that hole. But the struggles you have on the Mac will be creative, they will be musical, and there will be creative musicians who are fellow users and can help you out. The struggles on other platforms for musicians are often technical — you struggle just to get a timing or stability issue worked out and get the basic functionality that every single Mac user already has when the system comes out of the box. It is one of the most embarrassing things in computing — the lack of music and audio infrastructure in any operating systems other than Apple's. I wish it were not so — I would love to be able to tell you that any computer is good for music. But only Apple has ever made music and audio a priority on their systems. Only Apple built the infrastructure. Only Apple systems come out of the box as functioning music studios that you can then customize while maintaining that existing functionality.
Keep in mind that recording audio is a really high-stress computing task. You have to plug-in to audio interfaces with 96000 frames per second timing, you have to plug-in to 20 year old MIDI instruments, you have to run the CPU's under heavy load for hours and hours and hours without every crashing. There cannot be any crashes or you lose takes. No crashes at all can be tolerated. In many years in music, I've only ever seen Macs, iPads, iPhones do that amongst general purpose computers. To get that reliability otherwise, you need dedicated hardware.
I've worked at a few studio complexes where almost everybody is on Macs and a few guys for whatever reason are on PC's. It is embarrassing for Microsoft and PC hardware makers to see the difference side-by-side. You see a band of hardcore stoners who don't have a high school diploma between them and have barely ever touched a computer go into a room and successfully record their own original album with a $500 Mac mini, and next door to them is a guy who has a college degree who is paying another guy with a college degree $500 to get his $3000 Windows PC setup to stop crashing during recordings. Again and again, over and over, I've seen this. Don't be that guy. Don't bring a typewriter into a music studio and try to hook it up to a piano.
One other thing is there are Mac-based tools for mastering for iTunes, making iTunes LP, and other post-production music-related tasks. You will miss those things on another platform.
Finally, you said it wasn't important, but the Mac is also more open source than 99.9% of its competition. You can get CoreAudio, CoreMIDI, AudioUnits, and so on without giving up Perl, Python, PHP, Ruby, Apache, BSD, WebKit, LLVM, Clang, and so on.