My experience with 79/80 and 89/90 has been such that I would pay the extra.
On 79/80 (Carolinian), the coaches are Amfleet 1, 80 seaters, and the BCare hte 60 seater Amfleet 1. In that case, BC is clearly more roomy, plus, there's a better chance that the crowds will be less and you'll be able to have 2 seats for yourself. On my last trip, the coaches were packed solid on a warm day, and despite the A/C, the cars reeked, which wasn't the case in BC.
On 89/90 (The Palmetto), 60 seater Amfleet II coaches are available (though they have an Amfleet I coach, too, for people going shorter distances), and the BC is the same 60 seater Amfleet I. In that case, BC doesn't have an advantage in roominess of the seats, but I still prefer it because it's usually less crowded.
In general, I don't bother with BC on the NEC, unless it's a real crowded travel time and I want to increase the chance I can sit with my traveling companions. Maybe for a trip was WAS to BOS. And if I'm taking 66/67 overnight, I'll spring for BC, as that train has the 2+1 seating, the seats recline further, there are the legrests, and the windows have curtains. The same is true for the Vermonter. On the other hand, the Pennsylvanian abandoned the 2+1 club BC seating for a 60 seater Amfleet I, while the coaches are (mostly) 60 seat Amfleet II, so I think there's no advantage to BC unless you're traveling during a busy period.
In my mind, one of the main things you're getting from an accommodation upgrade is less crowding. The other stuff is just icing on the cake.