A bit of advice, I can't answer your question though to be brutally honest.
Don't make it so a donator automatically gets 2x experience, add something into the donor shop perhaps a ring or an amulet that allows players to gain double experience while wielding, it's a great way to get some donations, me being a huge skiller on Runescape and RSPS(s) loves the idea of 2x experience items.
It's all up to you though, I can only give you insight.
Best of luck.