Looking at some of my logs for 25m HMs - the T12 2pc is worth 350-450 mp5. Its' value dropped somewhat in 4.3, because it isn't always the right decision to use Holy Shock on CD 95% of the time anymore; sometimes you need to do HR-HR-HR-LoD for burst, and we have other ways of generating HP.
T13 4pc is very dependent on how you use Divine Favor. If you use DF for times of burst healing/HR spam, you get quite a bit of regen from the 2 piece. If you can get 8 Holy Radiances off during the 15 second duration of the 2 piece buff (not unreasonable given that you have a 20% haste buff at the time and are at 1.5 second cast times typically on HR), the T13 2pc is worth 455 mp5 with Divine Favor used on CD. If fight lengths allow you do use Divine Favor more than once every 3 minutes - i.e., a 3.5 minute fight lets you use it twice, the mp5 value increases.
TLDR, I think the two set bonuses are pretty close in value - at least on a fight where you spam heavily with DF up. The 4 pc T12 bonus is terrible; DL, FoL and HL have dropped from being 40%+ of our output to typically under 10% in 4.3. You may want to consider going with both 2 piece set bonuses, at least until 4 pieces of 397+ tier are available to you.
