This is a question lots of people asked. In the end, you can equate the comparisons:
At lvl 80
PROT
6% Heal increase (~ +180 SP to a 3k FoL, + 360 SP to 6k HL). This obviously will scale as you level.
30% entire raid damage reduction for 12 seconds every 5 mins (this is HUGE)
BoK
3 min Hand of Protection
RET
10% reduction of Holy Shock cost
8% Crit (176.64 Crit Rating) (Equates on avg to +125 SP to a 3k FoL, and +250 SP to a 6k HL, but don't forget the mana return on crits)
3% Crit for Raid (i.e. all damage dealers, not healing)
Imp BoM
The thing is that if another Pally has Imp Dev Aura, then that talent is neglected. So you will bring your 30% raid damage reduction every 5 mins, Kings, and a reduced CD Hand of Protection.
In a nutshell, if you are the only Paladin in the raid, then the Holy/Prot build will be awesome. If someone else (i.e. Prot Pally) brings Imp Dev Aura, then Holy/Ret seems better to me.
EDIT - Of course, these are just my personal opinions. I have not played in beta, so I can't confirm or deny what will actually work better.
Edited, Oct 22nd 2008 2:36pm by YJMark