just want to chime in with a testimonial if there's anyone else on the fence and in their mid 30s it might help you out
skated formula 4 99 duros for forever, when the hype around the bones x 97 came around i got some and really liked them and have been on those since.
while cleaning i found some unskated spitfires and on a whim decided to go back to those, skated street, and the next day my feet/ankles/lower back were absolutely killing me. put the bones x 97 back on and the difference is night and day
i never really thought the difference between 99 and 97 duro would be noticeable at all but it is. also you can skate longer because the wheels are noticeably faster, so you push less
everyone puts graphics in anyway so just get em your lower body will thank you
F4 99/97 vs X-form 99/97 is apples and oranges - it's more about the formula than it is durometer...each camps 97/99s skate totally differently than each other. I was riding a few setups over the weekend for some testing/madness one had conical 99a (black thane), another with ricta 101a, another with x97 and ANOTHER with x99s (which I swapped between white v1s and black v6s, that mad).
First, I was surprised how not chattery the 101 rictas were, I liked how they felt with forged plates. Weird. I skated those first / for a while to sort of set my body and expectations up for the softer wheels. I really like how hard they were, but they don't slide as well as stfs or f4s.
I've been on x97/99a for so long I am super used to them / they were fine. I'm finding that the well seasoned x97s slide much better than the x99s on flat ground (powerslides and reverts and especially on super chuck terrain they just glide), but the x99s perform better on slidey/grindy tricks or smoother terrain - today I went to a rough ass spot with 54mm x97s and those fuckers broke free while pushing to get speed (like pushing too fast and you get front foot speed wobble), I had so much weight front foot that the back was too light and just vibrated free).
The 99a spits...I don't know if the wheels are just 'beat' (or because I came straight off the rictas) but they felt like total shit...soft and slow, which was weird as I had put these on weeks ago after being on x99s for a while and they felt really hard...took them off and put on some semi-fresh white spit 99a lockins and it was night and day, they felt like 101s (and also heavy).
As for saving the knees, yeah man, 97s, even x99s...even the black x99s, which feel harder to me than the whites, will have you skating longer/with less fatigue/soreness the next day.
Just choose the right wheel for the job...but it's safe to say you can get away with x97/99s anywhere you are skating any 99a, and definitely places you wouldn't want to touch with other brands 99a.