The 25-31 is 31 inches tall, and is tuned to 25hz. I believe you can have it tuned at the factory down to about 22hz. All the PCi's have the same power and features.
How low you want to go ends up being the question.
I think the 20-39 offers the best of both worlds. It appears capable of handling nearly anything that a movie soundtrack can dish out. Except for the lowest of the low pipe organ music, the 20-39 is more than enough for music.