If you are going to use a subwoofer, I would get Ultra Bookshelfs for your mains!
You are giving up sound quality if you go with the Primes and the sub is going to cover the low end.
The Beauty of SVS is their no-risk return policy (which includes return shipping, so really, no risk)!
The Ultra uses excellent drivers and will do anything you could ask for to well below the 80Hz crossover point.
The Ultra is their top of the line speaker and their will be more detail than the Primes can offer.
Just to share an experience - I have been totally convinced that big speakers sound ... bigger! However, I had a friend come over to do the same comparison, but blind (he had no idea what the speakers were or looked like). He decisively preferred the smaller speaker! I asked him at the end of his evaluation if he had any sense of size or scale associated with the speakers and his reply was "absolutely not"!
I listened some more after this and found that even just closing my eyes when switching back and forth led me to realize that seeing the extra size influenced my perception. I always recognized better highs and more detail in the mids from the smaller speaker, but (with vision) the scale of the larger speaker (this "large bookshelf" had 4 each 6.5" mid-woofers and 3 each tweeters) seemed to better envelop me in sound which made it a hard choice. Realizing that the visual "size" factor was an illusion, made for an easy choice!