There has been lots of math stating the 4pc T12 is less desirable than what you are making it out to be. Use
www.treecalcs.com, input your gear and it will tell you how minimal the 4pc really is. It's not 2 swiftmends, swiftmend gives efflo, the extra proc is just an extra heal.
The 2pc T13 is amazing..I'm not sure if you really know what it does. It can save you so much mana, think about it..You're about to go into black phase on Heroic Zon'ozz, you're at 70% mana and power torrent just procced. You go tree-form, instant RG for Harmony, pop jaws of defeat, innervate and blanket your group with RJ for tons less mana, at the end of that intense phase, you'lll be sitting at around 80% mana. So no, you are wrong the 2pc T13 is absolutely amazing. You need to check your gearing/prioritization.
DI really isn't a toss up. If there is no one else it can go on, fine. But on a Druid, unless you're at the haste cap for it already (I believe its 916) then it's a waste. Yay you hit a few more ticks (theincbear.com/math/resto-haste-breakpoints). Put that DI on your mage, he can reforge some haste away for crit and do more DPS. DI doesn't really help you heal more, you gain some healing ticks that aren't really important. Trying to get more haste to reach those high breakpoints will reduce your mastery, crit, and over all healing.
If you're not doing hard modes, then it doesn't matter. Do what you want take DI from whomever you want, it will not matter.
Source:
www.elitistjerks.com
Edit: Vanyali, when I look at your armory, spec and reforge prioritization I can tell you are on the wrong track. Please stop posting mis-guided information, and take a look at what you're doing. This forum is meant to help newcomers, and people who post wrong information confuse these people who are trying to better themselves.