Which ring should I buy? sterling silver ring with rhodium finish or sterling silver with platinum finish?

I'm looking to buy my girlfriend a engagement ring for Christmas. She's not the fancy type and she refuses to accept an expensive ring, I've found to beautiful rings but I'm not sure which finish is better. .925 sterling silver with rhodium finish or Platinum finish over Sterling Silver? Please help me! I'm sacred to order a ring that will tarnish or give her finger green, help!!