So first some basic, honest disclosure - I am a 1982 Deck Graduate; my DS is 2012 and a "Shopper". My short answers to your questions are:
"1. What are the key differences between the experiences of a deck major and an engine major while at KP? "
Assuming we are talking about the comparison between "straight deck" (a major in Marine Transportation) and "straight engine" (a Marine Engineering) major vice a comparison of either of the other two "Deck" and either of the other two "Engine" majors: From he beginning of Third Class year on, the "Engine" major takes a lot more math and engineering courses, this is a course load that many, if not most would say is more academically strenuous. However, the "Deck" courses aren't "cake-walks" and usually involve more writing and research papers, etc. They are basically business courses. In both cases - "straight deck" or "straight engine" the primary focus is initial preparation for entry level positions in the maritime/transportation industry. Also the number of days required at sea to sit for the "Deck" license is more than the "Engine" license so that can drive certain aspects of sea year in that the Engine major MAY have a little more time between ships and that can enable they and the ATR to be a little more selective in their ship assignments, though the requirement for Steam time has been an issue in the past.
"2. What are the key differences between their experiences at sea?"
I assume you are talking about cadet shipping but there are also differences when sailing on your license. As cadets, engine majors experience seem to me to be even more a function of the quality of the other Officers on the ship in that department than it is for we "deckies" - I say this because it seems to me there is a broader range of differences between ships and ship systems/operations. From a deck perspective, I feel for example the differences between a 6 year old 6,000 TEU container-ship and a 2 year old 10,000+ TEU box ships are far smaller than the differences could be from an engineer's perspective as the ship's systems designs could be entirely different.... When sailing on your license following graduation the differences can and usually are far greater in today's Merchant Marine where the engineers may not stand regular watches after hours while the deck officers obviously have regular bridge watches 24/7 while underway. Pay-wise they basically have similar ranges though generally the top pay for Masters is a couple of % (literally 1-3%) greater than the top pay for Chief Engineers - of course the Master has a little more paperwork and license risk than does the CE and that is, at least theoretically, the justification for that difference.
"3. Does KP100 provide an accurate picture of both tracks?"
Well this is obviously a question, whose answer is subjective to a fair degree but mine is - to the extent any single course can provide you a basic understanding and preview of a 4 year curricula, yes it does a good job of exposing you to the basic, core differences.
"Any other info pertinent to the topic would be appreciated. Thanks for your help."
Relative to inital opportunities immediately following graduation, in times of tougher economies, I'd agree that engineers likely have more choices. That said, the differences both in terms of choices and in maximum economic potential pretty rapidly converge within 5 years out (again only my HO). Also, if you look at the recent graduating classes both 2009 and 201 graduated into what I'd call tough economies and from what I can tell they are doing OK. Certainly not as well, nor having as easy a time of finding jobs, they like, as the Classes of 2006 - 2008 but in comparison to their peers from most "regular colleges" they are doing very well as a group. (I say as a group because I'm certain there are several out there reading this who have personal experiences that differ and I think their opinions and experiences are pertinent and valuable to this conversation.)
Looking at my Class, now that we are 28+ years from graduation, I can point to numerous examples that would say regardless of whether you are a "Deck" or "Engine" graduate, your prospects and max earnings potential are high. For example at the top end of my Class economically I can point to 4 guys who I know have net worths of 9 figures (and possible more), all of whom made their money following graduation - 1 was Engine, 2 were Deck and 1 was "Dual". When we have reunions I can tell you that in all cases at all levels of accomplishment there is a solid mix, and I think our graduating class was like 60/40 (Engine/Deck).
So my advice is major in what interests you, when you graduate pursue a career you'll enjoy, that way you'll likely be better at it than one you're only approaching as "a way to pay the bills". In the end you drive and interest in your chosen career; along with the other foundational elements you'll leave USMMA with, will likely position you to achieve more and reap greater rewards than "the average bear". Then in 2019 or 2044 when your DS looks at where he is in his life and where he wants to go, he'll likely be both happy and successful regardless of whether he went "deck" or "engine". Oh and don't forget to tell him when he's one of those wildly successful guys, the best way to show off to his USMMA Classmates is to donate to the USMMA Alumni Foundation - often