Acela is newer, and, as one of my friends put it, is "very Star Trek." (I didn't know you could use "Star Trek" as an adverb, but he's an English teacher, so I guess it's OK.)
Acela's benefits, at least as compared to Northeast Regional:
*Speed (10-25 mph faster at the most, and more often, the same speed)
*Fewer stops
*Larger windows
*Brighter, cheerier interior
That's really all.
Acela first class gets you all the above, plus:
*2x1 seating (some rows have tables)
*Free hot meals
*Free drinks (including alcoholic, IIRC)
*Complimentary newspaper
It's very nice, but again, it's much easier to justify if someone else is paying (or if it's a once-in-a-lifetime thing).
Regional business is much more variable. It gets you complimentary non-alcoholic beverages and could get you any of the following:
*Half of the Club-Dinette car, with 2x1 seating and faux-leather seats
*An Amfleet business-class car with 2x2 seating but slightly more legroom
*An Amfleet I coach car with the word "BusinessClass" taped over the word "CoachClass"
If it's the first one, it's definitely worth it (though the only train that reliably has this is the overnight one, trains #66/67). If it's the second one, it's probably not unless you're really tall. If it's the third, it's definitely not worth it, as the only thing you'd get is a free soda and a slightly more upscale crowd of traveling companions (but probably fewer screaming babies).
If you get a Club-Dinette car, you can always attempt to upgrade on-board--though I'm not sure if you can do that on NEC services. Otherwise, save your money.
I'd concur with the above recommendation to do Acela one way and Regional the other. That way, you can try out both services to say you've done them (and make the decision for yourself if Acela is worth the premium). If you do that, I'd probably go ahead and splurge on Acela F--just once, though.