Keep in mind your bootfitter knows what your feet look like, and knows exactly how “big” a 22.5 is on you…No one on this site (unless your bootfitter is a member here), can say the same. There are a lot of absolute comments here that I do not think should be as definitive…there are always grey areas in bootfitting and ski gear in general. The perfect boot needs to match you foot’s anatomy, and your skill level…but there’s a lot of give and take.There's definitely a very frustrated side of me that is inclined to listen to the park city bootfitter that suggested I use a 22.5 for a few years as I continue getting better, then look into junior race boots from there. At the same time, there's so much incredible experience and advice on this site that I can't discount either.
Do you currently have the S/Max 90? Or was it recommended by the fitter, and now you’re looking for a deal?
Back to your original question…120 to 100 in modifications is a huge step, if we were talking 120-110, I’d say take the deal and give it a go. If you were recommended to go with the S/Max 90, the 120 should have an identical fit, but I would probably steer clear of something that stiff. Have you asked your bootfitter what they think? That might be the best place to start, considering they would be the one doing the work on the boot.