I was at about 2100 sp, 25% crit, and 1100 mp5 (about 500 while casting + another 150 from procs not counted) with only inner fire as a fully geared holy, but when I recently switched to disc I lost a lot of spellpower. My spellpower dropped by around 300, but I find that regen isn't as important with disc. I can spam heal for a 10-15 min fight like Sapphiron and not even go below 10k mana without using pots, shadowfiend, etc whereas with holy I would go oom easily. I'm finding that disc heals for about 60% of holy with the same equip. As disc, I think it's more important to stack spellpower and crit as you lose so much sp. Haste also seems more useful for disc since you have so much mana to play with, but I would worry about it after you stack enough of the other stats.
As holy I stacked up on crit and spirit the most. In raid fights where speed of the heal delivered isn't critical, I find holy to be a much better spec, but in fights like Kelthuzad with the frost blocks, disc's fast heals are superior. If Blizzard allows shields to stack, disc will be a very strong raid healer on par with holy even with the lower healing output, but it's currently being gimped by some design flaws. I would really love to see a glyph to shorten the cooldown on penance to 6 seconds when talented.