Actually a very good question. The short answer is that the Northeast Regional is just fine. You can get a regular coach seat at a decent price, especially if you book early. You can sit in the quiet car (shhh), or even upgrade to "business class" for a nominal charge, which gets you a slightly larger seat, footrests, a curtain on the window and *one* free soda, whoopie.
The Acela looks pretty, but they were never able to take advantage of its theoretical ability to hit 150 mph. There is one short stretch of track in Rhode Island (just before Providence, by coincidence) where they get it up to speed just because they can. The rest of the time it lumbers along like any other semi-commuter train. They make the schedule shorter mostly by skipping smaller stations. If you want to save 30 minutes and ride on a prettier train, by all means go for it, but it isn't necessary.
Acela "First Class" is actually pretty nice. You get a significantly larger seat (but not lay flat, sadly). You get all the alcoholic drinks you want and some sort of actual meal included. But it is way more expensive and really only for people on generous expense accounts.
Let us know if you have any specific questions, but you won't go wrong on a regional. Once you get past New Haven there is some really beautiful scenery, mostly beaches. Get seats on the shore side.
