REL Vs SVS for 2 channel music listening

I have a REL S5 and couldn’t be happier with it. When I purchased my REL a few months back I asked similar questions as you and considered the SVS as well. In the end I learned that one of a sub’s most important responsibilities in sound reproduction is to create a sense of space. The REL’s drivers are very fast and integrate seamlessly with my PSB T3’s and help to create an enormous and well integrated sound-stage. I love what it does with 2 channel music as well as movies. REL's philosophy and approach is a little different than other speaker manufacturers. I'd suggest going on Youtube and searching under REL subwoofer. Find a video where John Hunter, CEO and Chief Designer at REL talks about the REL philosophy. There are several very good ones.

Good luck with your purchase.

Rel makes fine subwoofers, no doubt. They also look good, but are relatively expensive.
For a while their unique selling point was that they connected at speaker level, and without a high pass filter for the main speakers. Hence their typical low pass crossover point had to be and was relatively low, with as little impact on the main speakers as possible.. These days, many other subs can do the same.
To be honest the sound quality of a sub is largely determined by the room. 'Speed' for example is an acoustic property and not as some seem to think a driver issue. Just look at REW waterfall graphs. Room modes are a nasty problem, only really solved by the combination of multiple subs and room eq.

Is there any advantage of a 12 inch woofer vs 10 inch? The Rel t/9i has a 10 inch woofer 300 watt amp. The SvS has a 12 inch woofer with 500 watt amp. 

The Sumiko's actually look like nice subs. Yes, they seem to be a REL copycat (Sumiko use to distribute REL).

In very broad terms the larger the driver,  the deeper it will go and the louder it will play without distorting. Conversely in very broad terms, the smaller the driver the more dextrous it will tend to be but will not go as deep. If you take a look at the entire REL line-up,  I believe they have 9 subwoofers total. 

Part of the REL philosophy is to find the subwoofer match for your speaker system and listening area. I'd suggest heading over to a shop that carries both SVS and REL and listen to them in a 2 channel system.