First thanks to all the fuckin idiots who posted their dumbass comments. Way to waste your time and our fellas.
Xen, I'm also trying to decide which Special Ops I'd prefer after college. The most important thing to remember is that each service has different jobs and they are all the best at what they do.
While they each have specialities, they also each get some direct action missions (except for Air Force). I'm sure some units get more though.
Here's a quick summary of the units and their specialties:
Army: Rangers, Special Forces, and 160th SOAR(A)- Rangers specialize in quick reaction and assault forces and airfield seizure (basically light infantry). Special Forces specialize in unconventional warfare (Their training is actually much broader than that but that's what they're most known for). 160th SOAR(A) is the helicopter unit that specializes in covert insertion and extraction. They are badass.
Navy: SEALS. Specializing in water borne activity but with new emphasis on land warfare due to iraq and afghanistan. They're the first to be called when they need someone to insert by water. Many have complained about not getting enough missions in Iraq and Afghanistan. Still, if you make it through the initial trianing alone (BUD/S), you are a bad motha fucka. They get to blow up a lot of shit and use just about every weapon possible.
Air Force: SOS unit specializing in infiltration/exfiltration and close air support. STS specializes in combat search and rescue and air traffic control along with close air support. I don't know much about these guys.
Marines: Force Recon - specializing in direct action and special reconnaissance. You must serve in an infantry unit in USMC before you're allowd to join this unit. Haven't heard much about it but I'm sure they're pretty fuckin tough.
With the Army, there's always your shot at making it through Delta Force selection. These guys are probably the best we have in the military. They specialize in anti-terrorism and are the first to be called when there's a high risk situation that needs to be taken care of quickly. Rangers and Special Forces have their shot at Delta.
The Navy has DEVGRU, which chooses its men from the SEALS. This used to be SEAL Team 6 (I think that's the right number, could be wrong) which was created by your man Richard Marcinko. They changed the name basically to update their image that was carried over from Marcinko's legacy. Obviously some bad motha fuckas.
"Masters of Chaos" by Linda Robinson. Great book on Army Special Forces with details of their missions and training. I'm finishing it up now and it's worth the read.
"Inside Delta Force" by Lee Haney. I picked it up for cheap at barnes and noble and it was worth reading. Not too detailed (OPSEC) but enough to get an idea of what these guys are doing. For instance, he talks about being sent into various countries for intelligence gathering and sniper missions.
"Down Range" by Dick Couch. Haven't read this one but it's an account of what SEALS are doing in Afghanistan and Iraq. I've read "The Warrior Elite" and "The Finishing School" by Couch, which are both about initial SEAL training, and they were great. Couch is an ex-SEAL who has probably the best access to information on them. Great reads.