These days there is not much between them as they now operate pretty much the same, (Although the 2 separate factions will claim there is, even when in reality there isn’t) however, the MAC is more expensive for the same performance but does have a premium feel. (Upgrades are a bit limited though)

Look at what software you want to use, and then check to see which platforms it is designed for, (Most music software will work on both) however, be aware that Apple will drop support for older software even if it is still popular to suit their purpose, whereas Microsoft still supports a lot of the older software.

Get a computer assembled for your specific purposes (Most online shops have a custom build section where you can choose the type of system you want, (Music in this case) and vary it to suit your needs)

If you have lot of older Windows software (Or love free stuff) stick with Windows as it probably won’t run on a Mac, (Even with a virtual PC install) so you will need a dual boot system, (You boot into either Windows or Mac at start up) with the Apple Boot Camp software installed.

Take your time choosing, and always allow for future upgrades. (Don’t buy a minimum spec computer to save money)

Just noticed your recent edit, so go for Windows as it will allow you to run the software you own OOTB.

Have fun

Bill