Disclaimer: some would say Kara/Heroics do not in any way count as "end game". I'm not one, but you have been forewarned :)
Tanking: Paladins generally will have lower single target threat, but on multiple targets, no other tank can compare. Period. There are places where Paladin tanks have a very strong advantage over Rage tanks, but there are also places that a Paladin will have a very strong disadvantage. If you're guild knows how to properly use a Paladin tank, you could be a great asset as one. Problem is not everyone knows how to properly use Paladin tanks :(
Healing: Sure Paladins can keep up, quite well actually. Single target healing is where a Paladin healer shines over other classes and the Paladins ion my guild consistently top other healers.
This of course all depends on how good you are as a Paladin, but assuming you are skilled, you can fit either role quite well.