What is an ELT?
An ELT stands for "Engineering Lab Technician". These guys (and gals) are responsible for the chemical and radiological aspects of nuclear power operation. All ELTs are selected from a pool of MM's (Machinist's Mates) and are sent to further schooling. Because ELTs are derived from machinst's mates, they do have mechanical skills in addition to their Chem/RadCon skills.
What is the difference between an ET and and EM?
An ET stands for Electronics Technician. And EM stands for Electrician's Mate. The ET's are responsible for (as you might imagine) the electronics and computer systems associated with the nuclear power plant. Most often, these are found in the reactor protection system. EM's are responsible for the electrical power generation and distribution. Motors, generators, circuit breakers, etc fall under the cognizance of EM's. There is considerable overlap between the two ratings, and they do attend many of the same classes and qualify many of the same positions.
Is it easier to move up in rank in either category?
Moving up in rank often has more to do with availability than anything else. You are only competing against others in your own ratings (ET, EM, MM, ELT), so you won't have an advantage over other ratings. That being said, ET's and, to a lesser extent ELTs, are really struggling to fill their manning requirements and so there's more opportunity for advancement to higher ranks as an ET or ELT (at least right now).
I scored nearly the same for all sub categories on my asvab, I don't have to take the nfqt or whatever the heck that is, so how will my rate be determined?
Should you decide to join the navy, you will go in under NF (Nuclear Field) and report to boot camp. Within a couple of weeks, all the nukes in your division will be herded into a small classroom where a master chief nuke will explain the different rates to you. At that time you will fill out a sheet of paper REQUESTING which ratings you want. They will compile all the forms together, look over their required numbers, look at your scores, look at your requests, and try to make it fit as best they can. The "Needs of the Navy" takes precedence over your personal requests. I asked for MM, and got my 2nd choice of EM. Looking back, I wouldn't want to be one of those dirty knuckle-dragging poo-flinging monkeys anyway.

Is it true that the chiefs determine your rate for you?
I don't know who makes the final decision as to what rate you get, but it wouldn't surprise me if it's that master chief at boot camp who does it.
When you swear in at meps, do you prioritize which rate you want?
No, MEPS only cares about getting you processed and into the system. Your rating will be decided at boot camp.
I am married, with one daughter. I'm 24, getting ready to swear in in June. Is it mandatory to live on base during school? My husband is a carpenter and we need a workshop for him to work and store all his generators, tools, air compressors, etc, so this is probably not a good situation for us, it won't be all too great trying to fit all that in a small apartment on base, we really need a house with a large garage off base.
When you finish boot camp, you will first be sent to Charleston, SC. As soon as you arrive and get checked in, they will allow you to go home and pack up your spouse/kids and move out to SC if you desire. I'm not married, so I'm not famliar with the specifics of this process, but I'm sure the navy will compensate you for relocation. As far as housing arrangements, I don't think it's going to be a small apartment. I think it's actual houses (with garages). There may be a waiting list for on-base housing. Alternatively, you could get a lease for a place out in town, but it adds complexity to an already stressful situation if you are living out in town. Honestly, I think you woule be best getting on-base housing. I don't think it will be as cramped as you think it will be.
I hear mixed stuff about bonuses, pretty much that it fluctuates somewhere between 12 and 20k. any way to determine before going to meps? i can always wait until a better month to swear in, might as well make more money for being patient.
12 to 20k?! Wow, times change. When I signed up 7 years ago, it was between 8-12k. The initial signing bonus is usually dependant on when you are scheduled to ship out. They try to adjust the flow of new students so there are no fluctuations. August is the most popular time for people to go to boot camp (high school seniors graduate, enjoy their last summer of freedom and then head off to boot camp). December, January and February are the least popular (it's a little chilly up there in Great Lakes, IL). So I would recommend talking to the recruiter and find out when would be the best time to ship out.
Is there a waiver in case I turn 25 before bootcamp? I hear that you must not reach 25 before ship date. there are waivers for everything out there, anyone with personal experience in this matter, please help. I turn 25 in November.
Where did you get that number? 25 seems awfully young for something like that. I know for a fact that there were people MUCH older than 25 in my nuke school classes. If there is a restriction on people 25 and older, I have absolutely no doubt you would be able to get it. Even so, I think you heard wrong, because the age limit is probably closer to 35 than 25.