Holy is healing damage taken and Disc is preventing damage. Try both and see which one you like better. I leveled up to 70 being holy and then switched to Disc and liked it more.
Disc would be the easier choice, it's far less gear dependant and less mana hungry which would make it less of a headache.
However Disc is far less interesting than holy...sometimes being challenged can be more fun (certainly is for me) and you might enjoy doing it slightly harder but having potentially a more powerful spec. Then again if you haven't played disc before it might be as much fun until you play that for awhile... at least I enjoyed being disc at the very beginging of LK when everyone rubbished it and when 3.1 came around I was sick of it and it became the best thing since sliced bread.