I have played both shadow to 60 and holy to 60. Honestly.. I prefer holy and I have learned to hate shadow priests in shadowform. Unless they are pve specced. In my experience though leveling my holy priest took much less time. Groups finished instances quickly which alowed me to grind out more.

Also... It takes alot of skill to play a holy priest correctly in terms of solo grinding. If you want a challenge, horde spirit.