I am a (tiny bit) mountaineer plus other sports up there related (ski touring, climbing, paraglide etc), ad I've also tried psylocybin.

Psylocybin was by far the most intense experience in my life (and I've been to both births of my kids and cut both umbilical cords, that was intense), all of those few times I tried it, some 15 years ago. Had a standard dose in dried form that I grew myself from growkit from Amsterdam, but mixed with raw lemon juice, which based on some erowid reports dramatically increase intensity while making trip shorter. I'd say its actually a cheat - you get some of the effects of efforts that others take years or decades to achieve, ie via meditation. But effects can be random (ie based on your personality and unresolved issues), both good and bad. And you get absolutely 0 of 'character building' which happens only if you face actual obstacles in life and overcome them with your inner strength, or via sustained focused effort on some goal.

I can attest that instead of rather mild 6 hour experience (1 dose on non-empty stomach of almost 100kg male isn't that much), with lemon I've had maybe 2.5h absolutely brutal trip of dissolution of myself, ego, senses and everything else into mist of atoms, swirling in background music like hot steam droplets in wet sauna. I was laying in the bed, alone, with nice shamanic music in the background, eyes closed. Coming down back to reality one nanometer at a time, rediscovering my senses, body etc was very very interesting and pure happiness-inducing. Then afterwards an absolutely brutal headache, literally feeling I've burned every last energy molecule with that expanded mind, and now whole brain is spasming. Still worth it.

Whether it rewired something in my brain or changed something I cannot say, I didn't have any previous issues (and TBH if you have ones, stay the fuck away from these powerful substances, the chances that they will mess you up for long/forever are non-trivial. Rather work with the issues sober and come to this afterwards, or in future hopefully this can be done with professional supervision).

Mountaineering or generally coming close to death and overcoming the fear of it on the other hand is something you do sober, you can do every weekend without your friendly neighborhood drug dealer, and it just improves your health and happiness (not so much during the adventure if its intense, but definitely afterwards). I'd say proper climbing is distilled essence of mountaineering - just focus on scary parts, where you battle with your mind which just wants to give up dangerous stuff and go home. You can't reach the top if you can't conquer your fears. Now do it a hundred times during 1 evening, once of even few times a week. I've come close to death a few times, but those experiences don't have immediate massive effect, rather they are being incorporated one 'atom' at a time into your personality.

I'd say both leave lasting effects, but how do you want to measure subtle gradual changes in one's personality. Haven't touched mushrooms for 15 years, never had a bad trip but I feel they gave me what they could, and continuing is just about getting wonderfully wasted. For father of 2 amazing small kids this is absolutely useless, a bit of weed here and there is enough for me to process anything these days.