I can't imagine why someone would use open cell foam if it is more expensive. Open cell foam can absorb water. Even if they have a base pan, if water gets in, it is going to sponge up into the foam. At least that is what I've been told in the past.
On the contrary...I've got 3 HotSprings I'm currently dealing with, a 1998 Grande at a customers house, a 2000 Grande in my Warehouse and a 2000 Landmark behind my store empty.
All three have leaks, we think we fixed the one in the field 6 month ago however we've gone back several times over the last few months to re-foam it and the existing foam is still wet so we did an experiment… The one in the warehouse and the one outside have both been sitting for over 4 months without water yet both are still saturated in the cabinet.
Likewise we've fixed a few spas that had icynene this year with leaks and re-foamed them within weeks. The closed cell acts as better support but is much harder to work with in repair. It’s also harder to track down a leak in the closed cell vs the Icynene.