Welcome aboard ! 
I just went through the same dilemma with my daughters laptop. I don't use one for karaoke, so you may get a better response in the tech forum, but from what I can tell these are the major differences.
Mac -
No worries about viruses, spyware, malware etc... You don't have to have any protection for these.
PCs are now mainly Vista. You can still get a copy of windows XP and have installed instead though. Vista has shown to have a lot of problems. 
Mac also takes some getting used to because the formats are all different from pc, so if you've never used one there is a learning curve.
PC - Slightly cheaper still although Mac has come down a lot in price. 
Good luck with your decision. I ended up getting my daughter a Dell.
 
					
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