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I have both 54mm Classics and Radials in both 99 and 101 on boards.
The 99s are very consistent in the feel on everything, but the 101s are a bit more here and there in terms of slide, grip and everything else, so it is not so much that they feel harder and slide more, quite often feeling more grippy than 99s but sometimes being a little more slick when you do get into a slide.
Either way, Radial when new is a little bit more of an "all rounder" compared to Classics when new.
I like both, but I can also feel the difference - not a whole lot, but there is a difference there.
Overall I think if I could get 54mm Radials all the time, I might ride those over Classics, but they are somewhat hard to come by at times, so Classics are the easy fall back option, as I do prefer a round edge but a wider profile and riding surface.