I'm a CTR (Cryptologic Technician Collection) in the Navy, and I deal with both of themfrom time to time.
CTI (Cryptologic Techinician Interpretive): Your going to go out to Montery,Calfornia (I think) and learn a language for "A" School. You can choose pretty much any language, some that will benifit you more for advancemet are Arabic, Chinese, and probably Korean. You WILL be translating. If you go to Iraq, then you'll have Marine bodyguard(s) and you'll be taking with the locals, maybe even interegating people. Other wise you'll have headphones on.
CTT: Cryptologic Technician Technical): I'm not sure what they do state-side, but in the Middle East, they ride in the convoys and they jam the IEDs. The CTT's "A" School is in Pensecola, FL.
If you go either, you're going to get a Top Secret Security Clearance. The one that will get you the most money after you leave the Navy, CTI, because things are heating up between the US and China, so we'll be needing more Chinese Linguists. I would reccomend CTI, because a lot of the linguists I talk to really enjoy their job.
Hope this helps.