The only thing that matters is SPL, extension, and distortion. If the SI 18 can play low enough and loud enough and with the limited amount of distortion you desire in a simple cubed sealed box, I don't see a lot of reason not to take that option. Your upgrade path is easy, add another. Use a nice amp with DSP or active electronics, really a nice value for the money. Assuming the money isn't a factor then I guess you can flip a coin.
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."