I own one of each, a REL S2 and SVS SB1000 Pro, and I prefer the SVS. They augment a pair of OB speakers in a 16' x 21' room to great effect where you feel immersed in the sound.
As @soix mentioned, 2 subs better than 1. Easier to get good sound. 3 subs even better! Multi-subs are able to smooth the lumpy bass found in all rooms.
A while back I managed to borrow 2 ported subs to play with and found them a little difficult to place and that is why I chose the SVS sealed unit. I placed the SVS in the rear right corner and the REL about middle of left wall. Omnimic confirmed a better room response than the 2 ported designs and that was before any fussing over their positions.
The SVS has continuously variable phase adjustment and with the ability to change settings from the listening seat is a great help. In addition to phase there is also low pass, parametric EQ, room gain compensation and presets.
The REL ( it's the S2 I think) which I found used at a bargain price was placed along the side wall so I would have the option of moving it about for best integration because the phase is only switchable from 0 to 180 degrees.
Setting up these 2 was a breeze. With Omnimic guiding me I moved the REL backwards and forwards, each time readjusting the SVS then move REL again and further tweak the SVS and so on. Will now build myself a servo-sub to add to the fun.