Just another opinion from an ex-lifer:
As an ELT, I would wait until you have experienced the fleet, expecially if you are a submarine ELT going to a fast attack.
By this time, you've experienced advancement exams. In my opinion, the only reason to consider automatic advancement as the incentive is if you don't feel comfortable with Navy testing. On my three boats, I only remember one ELT who had trouble with an E-5 test, and that was only before I knew he needed some advice about how to prepare for the exam.
I don't think getting a C school is an advantage for you. In fact, I was looking at what I assume to be the current instruction the other day to answer someone else's question, and I didn't see any authorized C schools for ELT NECs. I didn't even see the Advanced Aux package that used to be available for MMs. I STAR reellisted just before the four year point, but never requested my C school. (I would have waived it to get back to sea, but that never came up when talking to a detailer.) I never met any E-6 and below ELT who STAR reenlisted and went to a C school, although now that I've said that, I'd guess someone will give their experience as a reply. (I say E-6 and below because although I knew some E-7s and above, the subject never came up.)
STAR C schools for nuclear trained personnel can be found here:
http://buperscd.technology.navy.mil/bup_updt/upd_CD/BUPERS/MILPERS/Articles/1510-020.PDFIt would be great if you could plan your career, as others have mentioned, to have shore duty for your last years in the Navy. You must consider that, just as the exceptions mentioned where the circumstances were particularly favorable, there are other things that could happen. Instead of getting tender duty, instructor duty, or being picked up for a degree completion program, you could get some needs-of-the-Navy, CNO-priority billet, like recruiting.
I'll mention this with the emphasis that I consider it to be an exception and not what the average sailor today would expect: I had finished my first five year sea tour and was deployed in the middle of a followup three year tour. At the two year point, I ended up with orders to recruiting duty to report a week after returning from WESTPAC.
(OK, now for the circumstances that are not typical and are explained in more detail for those who might think they are improbable. I might not have been on the detailer's good side because I had turned down instructor duty in Hawaii to go back to sea. Oh, never get on your detailer's bad side. When I turned down shore duty, my 1306 said "any boat not in the shipyard", so guess where he sent me? For some reason, PRDs were advanced Navy-wide by six months in 1988. My boat had received my orders in the middle of deployment, but had never told me because they were trying to get the orders cancelled or modified, apparently not wanting to have to replace the LELT and an EWS with short notice. At least that's what I a junior yeoman and radioman confided to me while I was trying to figure out why I had gotten a Welcome Aboard Package from Navy Recruiting in the sacks of mail waiting for us on the pier. I never got an official story since everyone was on standdown and had maybe one duty day on board before I left. In retrospect, I cannot believe that any other command would do something like that these days.)
Anyway, as unlikely as that scenario is, it is just to illustrate that there is always some uncertainty, so unless you are sure you want to be committed to the Navy, as opposed to just considering primarily the incentives for reenlistment. If you are looking at long term (8 to 20 years) the advantage I see is getting rid of that two year extension and having the time count toward the bonus.