When 2.3 hits I'd say ret. I'm already building up extra t4 ret gear just to test out the new buffs, looking forward to it. Soloing as holy sucks balls, did it to get my netherdrake and man did I hate every moment of it.
Prot is great for farming on crowded areas like the training guards on black temple etc, rather anoying and slow 1v1. Ret is alot better for farming motes or any single target grinding.
Soloing as holy isnt hard at all, just get some spelldamage gear and your set, have been holy since i dinged 70 and never had any problems.