It may just be me, but I'm not really that thrilled with the resto shammy t13 set bonuses when compared to the t12.
t13 2pc: After Mana Tide, heal cost reduced by 25%. The highest amount of savings that will net you is about 50% of your base mana. (By casting Healing Rain, Rip, GHW, GHW, Rip, GHW, GHW. Sum of base mana cost is 198% * .25 = 49.5% base mana.)
t12 2pc: Your Riptide periodic heals have a 40% chance to restore 1% of your base mana. Assuming you have Riptide glyphed and have hit the first hast breakpoint, your getting 40% of 8 ticks = 3.2 % of base mana per riptide. Now, if you keep Riptide on CD (which you should) on as many different targets as possible, you can cast 30 Riptides during Mana Tide's CD, netting you 96% base mana over the same time as the t13 set.
Now the 4pc bonuses are a little harder to compare.
t13 4pc: Increases the duration of Spiritwalkers Grace by 5 secs, and increases spell haste by 30%. Not going to lie, this is pretty fantastic. While I rarely find myself needing to move for more than 15 secs, having an extra 30% haste. The biggest thing for me is that it adds up to two ticks to Riptide and Healing Rain, four total HR ticks over the duration of the haste. However for the sake of comparing it to the t12 4pc, I'm going to use Chain Heal as a benchmark. Let's assume that you cast CH with 15% haste and it averages 20k healing total. With the extra 30% haste, you can cast an astounding 14 CHs during that 20 secs, totalling 280k healing.
t12 4pc: Your Chain Heal no longer consumes your Riptide on the primary target. Since riptide increases CH by 25%, you can pretty much spam CH to your heart's content here, throwing in a new Riptide every so often. With the afore mentioned CH stats you can pump out 8 CH's that heal for 25k in the same 20 second window. This amounts to 200k.
Clearly, in that 20 secs, the t13 wins out. However, as the t12 is a static bonus, you still have another 1m 40s to make up that 80k difference. Keeping up Riptide, it only takes 45 secs of Rip/CH/CH to make up that other 80k difference. Leaving almost another whole minute of CH that trumps the t13 throughput.
Now obviously the t13 has it's benefits too. It's set bonuses are more universal and so give you much more versatility in their use. Additionally, t13 will also win out due to the better stats on the gear, but its a little sad to know that I have to trade those stats in for set bonuses that offer weaker mana conservation and weaker throughput.
I realized my math was a bit off for the 4pc comparison. So, while it doesn't change things that much, I'm adding it for completions sake.
For all my calculations, I assume a 15% haste buff from gear, and a chain heal with a 10k primary target heal.
With a 15% haste from gear, and using WoA for another 5%, CH will cast in 2.07 secs and heal for 25.33k total.
With the t13 4pc, the cast time is reduced to 1.59 secs, allowing you to cast 12 during SWG's duration, for a grand total of 303.96k healing.
With the t12 4pc, the total heal increases to 27.83k. With a 2.07 sec cast time, you can net 9 casts in the same 20 sec window, totalling 250.47k healing.
That's a difference of 45.49k. Now, assuming Riptide is glyphed, you can cast Rip followed by 9 CHs in just under 20 secs (19.87 to be exact) gaining back 22.5k healing with the t12 4pc. So after 40 secs, you've made back all but 500 hp.