World of Warcraft Comp. Question Need Help?

Alrite on World of Warcraft my server Anetheron Horde my GM and I are doing 3's but were not too sure what our healer should be. He's an Unholy DK and im a Shadow priest we both have over 3400 res. and we got almost 1600 with a Holy Pally but as we got higher didnt seem to push further. Now we have thought about a Resto Shammy thats real geared or a Resto Druid. Myself personally want a Resto Druid because of their buff and the ability to have good hots but just not too sure. Does anyone have any thoughts about a good comp. for my 3's team? Also if you have a good comp and you have that specific thing and my GM and i agree with you would you want to server transfer and play on our team :P. We currently have a Disc. Priest but one prob. is our fears share diminishing return as hex and cyclone wouldnt but we cant push past 1500 with that but the Disc. Priest is over 3600 res. and been playing in the guild and BGing with us for a month or 2. Btw we are PvP guild so thats all we do. Any tips hints or comments are appreciated Thank You.