FWIW, I've been using Ilfosol-S for a number of years and like the qualities with the Ilford films I use it with. I only found out recently that it is ascorbic acid based but I don't know any more than that.(posted 8128 days ago)I would say I've run well over 150 rolls of 120 & 35 mm film and had zero failures of the kind that some Xtol users have reported. Because I tend to shoot film mostly in the summer, some of the Ilfosol I used had been opened for more than 6 months but it seems to keep well nonetheless.
I'm pretty sure that Xtol came out after Ilfosol because I was using Ilfosol before and I wanted to try Xtol and see what the hubbub was all about. My experience with Xtol was good but limited to 1 roll of Neopan 1600. I still use Ilfosol because it's cheaper and suits my mode of useage.
Cheers,
Duane