For beginners, is there a much a difference in training and diet regimen whether one's "end" goal is strongman or powerlifting?
Does training/diet for the two diverge only at advanced levels, or even for beginners/newcomers as well?
The very quick answer is that Powerlifting is more about standing in one place and moving a heavy weight through a range of motion (be it deadlift, squat or bench press). There is a certain technique element to these lifts that can yield large jumps in weight (i.e. arching your back more on a bench press so the range of motion / distance you move the bar decreases).
Also, Powerlifting requires one big effort for a single rep (though you do get multiple attempts).
Strongman is about moving big weights, sometimes rather unusually shaped big weights (i.e. Atlas stones). Unlike powerlifting, there is a locomotion aspect to a lot of strongman events. Picking up heavy "stuff" and carrying it for distance or loading it onto something is a common in strongman events (i.e.farmers walks).
A lot of strongman events are also done for reps, distance, or time. Whereas powerlifting is all about the total amount of weight. In a log clean and press event, you'll be given something like 75 seconds to clean and press a weight as many times as possible, which requires a surprising amount of work capacity / conditioning.
Also, powerlifting has competitions for lifters using powerlifting gear (squat suits, bench shirts, etc) and raw lifters. As far as I'm aware, strongman doesn't have the same restrictions.
Both sports have weight classes, though admittedly the most famous strongman events are open weight.
To someone starting out, I'd recommend concentrating on getting strong in the big 3 powerlifts as well as the overhead press, and also throwing in conditioning in the form of sled pushes / pulls and a lot of picking up heavy stuff and carrying it.