I just went through the great quarantine string experiment of 2021 and really went back and forth between Perpetuals and various gauges of Spirocores.
The Perpetuals compared most favorably to the 3885 Weich set of Spirocores, not the S42 set. I think I preferred Mittels to all, but my bass really disagrees.
I agree that Perpetuals are a bit more "polite" or well mannered than Spirocores, but they are much better balanced and once I got setup dialed in they were equal in terms of projection. There is that special dynamic with Spirocores that really nothing else has, but the Perpetuals sound very good and they feel much better to me. I think it's the rope core, but you can lean on them without them giving out or getting tangled up in them or you can kind of fingertip feather them and they respond well to both. I'm not the best guy to ask about arco, but my conclusions were when I got everything adjusted for whichever string set was on there, they were all fine for a jazz guy. Perpetuals are probably easier for a novice/medium skilled arco player to not sound objectionable on, but I could have dealt with Spirocores too.
I incidentally opine that this is the biggest issue I see with mixing string brands or even gauges from the same brand, going string to string with the bow I'm more likely to squeal. I seem to make some subconscious adjustment to the bow attack with whatever string set is on there and after a bit of daily practice, it's ... I'll stick with "unobjectionable". But, with some guest G string or something, even if both the D and the G are fine arco, going between them, I'm usually missing the adjustment. Not an issue pizzicato or I'm better and subconsciously making the adjustment pizz.