Penny,Personally, I think the Mac is the best option for working with just about anything, especially music notation programs. However, the truth of the matter is that whichever platform you feel most comfortable handling is the best one to use as they both work pretty well. You might try using your sister's setup and see what you think before going out to buy a Mac.
For the most part, the Mac OS is a bit more stable than Windows and add ons (MIDI interface, scanners, printers, etc) cause fewer problems and are mostly easier to set up. In the music department where I teach, we have 14 Macs and 2 PCs in the instructors' studios. The computer support staff averages one service call per week for PC users for either hardware or software problems. In the past two years, there have been no Mac service calls.
Another consideration is that you may be able to buy a whole new computer (Mac or PC) for about the same amount as you can upgrade your current system. I have not looked at the cost of PC upgrades recently, but on the Mac side, getting an older computer close to the speed and power of new one sometimes is more expensive than buying a new one.
Good luck and keep writing.
Jim Coull