Need help deciding about a new Mac

Get a new computer, use your current one as a backup and casual rig.

LAPTOPS
If portability is important, the 15" Retina MacBook Pro would be my recommendation. The standard 8GB of RAM is more than enough for music production but if you want to future proof yourself, upgrade to 16GB. The only downside to this computer is upgradability. Everything is soldered onto the motherboard and nothing can be upgraded (except for the SSD, but that's expensive).

If you don't care for portability, the standard 15" MacBook Pro would be great. You get more bang for your buck and allows you to upgrade to an SSD and/or upgrade your RAM after the initial purchase. Downside to this is that the standard configurations do not include an SSD.

DESKTOPS
The 27" iMac is the best deal, hands down. If you have the cash, spring for a Fusion Drive for added speed.

However, wait until after WWDC so you make the best purchase possible. There might be new products coming soon so don't buy now.

 

For music production, the "wider" the display, the better (lets you see a longer "timeline").

I'd suggest either a 27" iMac, or perhaps a late model refurbished Mini with a 27" external display.

 

I would look into getting a 2nd hand or refurbished Mac Mini as that should handle your needs. As you don't need a great screen for music production, just get a cheap 27" display.
Transferring licensed software will vary with each company but shouldn't be a problem.

 

refurb mini's are a really cost effective upgrade if you are simply looking for a machine that can take more RAM. Any 2011/2012 model can take a 16GB upgrade. Of course you have to get a screen if you don't have one laying around, but if you are producing music you might not have to spend a lot on one that suits your needs.