I don't have a very long time to go into a lot of detail but the basic jist of the two AoE tanking builds is this;
Both builds do extremely well in AoE threat scenarios. realistically the key difference between the two is threat style and survivability. The Frost build has a lot of damage mitigation talents and relies on the heavy-hitting Obliterate, along with Rime/Killing Machine or Deathchill procs to lay down the hurt with Howling Blasts that hit like a truck. Glyph Howling Blast and every time you fire it you'll be keeping your disease spread, so your rotation will get tighter as you enter your second or third iteration. Frost DKs also have the option to dual-wield and, in my opinion, become more modular, with High Threat (Slow/Slow) Normal (Slow/Tank) and Max Survivability (Tank/Tank) weapon itemization.
Unholy Tanks are a rare breed, they rely on magical damage and debuffs to hold threat, while at the same time providing utility to the raid with Ebon Plaguebearer. Key differences you'll find is the only AoE burst threat you have is Blood Boil, so you'll have to do more disease maintenance to keep your threat maxed, and, more importantly, a Unholy DK has fewer damage mitigation talents, you will rely a lot on Bone Shield to mask your squishiness. It will require intimate knowledge of fight mechanics to know when the best time is to pull up your Bone Shield and how best to time IBF and trinkets to keep your durability up when you're waiting for the BS CD to drop off you.
It's the general consensus that Unholy DK Tanks are the most difficult to play and play well. But, what the hell fun are we having if there's not some sort of challenge to our playstyle? Facerolling seems to be a stigma that surrounds us DKs, even in the tank world, and I get tired of it, so if I can find something unique to do, I go ahead and do it. There is a sticky in the Class Mechanics forum that has several posts of very good and very accurate information concerning Death Knight tanking, including Unholy Tanking. Check it out and good hunting!