Just to follow-up here, we took this advice and stayed at both for 1 night each last week. It's quite remarkable how different they are despite their proximity and being originally built by the same person. Zaborin feels quite modern (and hotel-like in some ways); if they had told me Shiguchi had been there for 50 years, I would have believed it. I'd give Zaborin a slight edge on hard product, but the service at Shiguchi was meaningfully better (no particular misses at Zaborin, but the Shiguchi GM was excellent pre-arrival and throughout). The biggest difference was in the food: Zaborin served a perfectly fine but not particularly memorable, mostly traditional kaiseki meal, while Shiguchi delivered a terrific modern tasting menu that was among the best meals of a 2-week Japan trip.

Net-net, I would recommend that someone with 2 nights in Niseko visit both, but if only 1 night, I'd say Shiguchi is the superior choice.