Is windows or Mac better for a minecraft server?

None is better than the other, Minecraft is Java based and they both run Java exceptionally well, some would say Windows is better but being Java based there's no difference. It's all personal opinion, I play Minecraft on my Mac is it is my primary computer, other games I use my Windows machine as a lot of games are not available for OS X.

Pretty much the same

Neither. Linux will give you much better performance. I've ran Minecraft servers for several years, and Linux is always the fastest and most stable.