I wish I was in your position to decide between these two
!! Those are both quite powerful and capable subs. Rythmik and SVS is like choosing between BMW and Mercedes.....they both will perform for the price, but its gonna come down to taste. Here are a couple of good descriptions of the sealed vs ported sub box:
"A sealed subwoofer box will deliver a tight, clean and accurate bass response and have a flat frequency response curve. In general a sealed box will be for listeners looking for sound quality driven system that is clean across the entire frequency band."
" Ported enclosures are tuned to a certain frequency range which is determined by the port length, width and diameter. The port can be in several configurations such as rounded or a slot which is narrower and taller. Each subwoofer will have a specific frequency they are designed to run at for maximum output."
From my novice audiophile knowledge I believe that sealed subs seem to be more accurate with less output where as ported subs are more about depth, low bass range, and output..
Not sure if this helps, but hopefully it will get you started. Tons of awesome knowledgeable folks on this board!
Just my opinion ( Id go with the SVS).