an update for all who contributed, as to how it went, in the end:

I took the Amtrak, having redeemed 3000 CO miles for the one-way ticket BOS-NWK, and was pretty pleased with it (saving ~$85 fare) -- although it almost seems a shame to use it only for this short segment given that any trip in the entire NE corridor costs 3000 miles (although I can't imagine taking it all the way to DC, for example!). It even allowed me to take home a glass globe centerpiece of flowers filled with water to NJ (from dinner the night before), which I doubt I could ever have done on the plane -- the amount of seat space on Amtrak is just great, and the stable floor to put the vase on! And handy power outlets to charge my laptop and phone and watch some movies.

Absolutely no regrets, and because of the mileage redemption I was able to book at basically the last minute (so allowing me to stay later after lunch) with no worries about availability or skyrocketing ticket price. The online booking tool gives you a reference number and I picked it up at the ATM-style ticket dispenser at South Station. Was also pleased to be entranced by the Solari flapping train departures board for a few minutes before departure. (see also this thread!

On the other hand, as some here said, it did take about 4h45m, which I wish could go by quicker. Someday, maybe Amtrak can go fast in New England... Plus, couldn't they speed up the 1/2 hour stop in NYC?

Anyway, thanks all for the advice!