What makes 8" rel better than a 12" svs?
The ability to play cleaner, faster and better higher up.
The SVS starts to loose its grip post 65hz when it comes to music, it integrates better with speakers that can play well down into the 45hz range, by that range I mean the speakers that can play that frequency at +-3db and not +-10. The moving mass of the 12" also makes it harder for it to keep up with a 6.5in driver higher up the range in terms of agility and speed.
I'll share my known's case here he used a lumin d2>bryston 4b3>SF sonetto I>svs sb3000, after using it for more than 8 months and after numerous efforts we had not been able to integrate the sub and the speaker to a point where the sub is not very evident. Its crossed over at 60hz where we found it to blend the best(to our efforts) yet the low end was slightly hefty and thick, one could easily makeout a sub as playing, the upper bass was muddy, it just lacked synergy. Finally after cursing the sub for all this while and putting it up for sale, one fine day he went ahead and got himself used monitor audio pl 200 II and within a week he loves his SVS to the core, the subwoofer just shines and disappears in the whole setup crossed over at 43hz. The bass was already good with the speakers what the sub added was more depth and space to the already good bass along with extension.
So all in all it depends to what caters to your needs there are things that REL is good at then there are things that SVS is good at. In your case where the 8" rel will be good at is speed, start stop accuracy, kick, easier integration at higher frequency, ability to completely disappear
VS the 12" svs on the other hand will have much better extension and dig much much deeper than the 8" rel (+-3db@20hz svs
vs +-6db@32hz t5i), will be at home in being used in a HT application, rumble harder.
Rest if I have forgotten to mention anything or if I have mentioned something wrong FM's can add word of experience and advice