I have the Zen Mini Mk3 with a hotrodded Zen Mini PSU in my northern (England) pad and a Pulse Mini with same hotrodded Zen Mini PSU here in the midlands. I compared them for streaming and they sound identical to me into dCS into ATC actives. I’m assured that the PSU changes mean that my Minis are giving me at least as good sound quality as the full-fat Zen and Pulse.
Innuos will no doubt continue to refine/upgrade both models as they research new components etc and there may well be an offset between implementing that in one and doing so in the other, but this will be due to stock levels of current versions as much as anything. I’ve seen no evidence that they are deliberately differentiating based on sonic performance for some reason and can’t imagine what business logic might compel them to do that...
Basically, the Pulse range is the Zen range without the internal storage; it’s an appropriate and cost-effective response to the obvious market demand for a pure streamer "without the cost and complexity of storage I will never use" as some would say.
I’d buy whichever suits with full confidence in its sound quality. Myself, I’m looking forward to moving up both ranges as funds allow.