I've read through the entire thread and haven't seem too many impressions from people who've demo-ed both the primes and ultras side by side, so I wanted to give my very quick impressions of them.
I've been using the prime bookshelves, center, and satellites (all black ash) for a little over a month now and I've enjoyed them thus far immensely. They look nice, image well, sound very clear, crisp, and neutral to my ears, and for the most part don't call too much attention to any particular frequency. I'd say they're a great all around set of speakers which can transition from HT to musical use with ease. My biggest complaint with them is that the treble is a bit sibilant/edgy at times, but this could be due to me being particularly treble sensitive. With that said, I truly love the primes and for the price, I think they represent the best speaker value I've personally heard thus far.
I recently got a pair of ultra bookshelves (piano gloss) off the outlet for $700/pair to see if they were a noticeable improvement to me. My first impression was that the piano black gloss looks amazing! I got them for $350 each due to light scratches that couldn't be photographed, but all I see is swirl marks if I shine a flashlight directly on them; no scratches. Either way they look fantastic and the swirls are only noticeable if shining a flashlight directly on them. I dusted them with a chemical guys microfibre cloth and aggressively rubbed a bit of the underside to see if it would scratch the finish (the microfibre cloths don't scratch my car's clear coat so i figured they shouldn't scratch the gloss finish) and to my relief, it didn't scratch. I'm not certain if the swirl marks I'm seeing are normal or from the past owner dusting them. Either way, if you get the black gloss, expect to dust them a lot as they're dust magnets and don't be too sad if they scratch a bit, because it's really not that noticeable unless it's a large scuff mark. As for the sound, I was pleasantly surprised. Surprised because from everything I've read to speaking with SVS reps, I expected them to basically sound the same, but be a bit smoother in the treble, more resolving, and slightly bassier. Tonally they are similar, but I feel the differences between the two is much more apparent than what you might think. The biggest difference, to my ears, is the bass presence and treble smoothness. The ultras have a more full bodied bass and sound a bit warmer than the primes. The ultra's treble sounds smoother and never was sibilant to my ears. As for the soundstage/imaging, both image incredibly well and I feel I could happily use them in stereo without a center for HT use, but the ultra's soundstage felt larger to my ears. I only compared the speakers using music playing through my denon in DIRECT mode (2 channel only), so I can't speak to differences in HT use, but the Ultras sound like the more "musical" speaker to my ears.
With that said, the primes are no slouch and I'd argue they're a better value overall than the ultras. I hope this helps someone who's on the fence considering the two. If anyone has any specific questions, please feel free to ask and I'll help to the best of my ability. I have a pair of PSA mt110 coming in Saturday which I could compare with if anyone is interested.