Here are some thoughts of 1st edition AD&D vs. GURPS based on recently running both of them.
I've tried to roughly organize these but they do jump back and forth as subjects intertwine.
I've heard it said that in GURPS, if you're taking damage, you're losing. In AD&D, if you're taking damage, you're also losing. More slowly, perhaps, and without the death spiral of "hit means less combat effectiveness" that GURPS can include, but you're losing.
And where GURPS has resistance rolls and checks to keep fighting, AD&D is just "0 HP and go down" and plenty of monsters come with save or spectate powers. You're sturdier in GURPS but have a death spiral. In AD&D you are ultimately more fragile but don't have a death spiral. You trade a possible sudden shift in the combat (GURPS) for a steadier but not safer drain of resources.
Not only that, if the enemy rolls well enough, you're hit. You can't really anchor a combat and fight off the foe except by not being hit as you kill those you hit. The lack of DR means your HP can bleed off very quickly, even if AD&D generally lacks the catastrophic hit potential of GURPS.
Unlike GURPS, where healing casters are limited by easily-recoverable FP and have access to flexible healing spells with fixed effects, in AD&D you've got randomly effective healing spells in limited quantities. Run out, and you can't just rest a short amount of time and get them back.
GURPS lets you use your skill to maximize your damage. You can use skill to hit vulnerable locations and apply your specific damage type to those that give you the best potential results. In AD&D, you just take what the dice give you.
This does make it even more critical than in GURPS to pile on and focus your attacks on a smaller number of targets. The fragility of foes (they also drop at 0 HP, generally), their ability to harm you with a single good roll, and their full damage potential until death (or at least unconsciousness) means you need to put them down. If you assume the best thing to do is spread your damage out so none of it is wasted (IOW two fighters attack two foes, "just in case you drop yours in one blow") you're taking a risk. In GURPS this pays off as well, but a wounded foe might pass out seconds later even if just left alone, foes might be reduced in threat by a good roll, etc. You can give yourself more leeway to fight with a foe knowing you can occupy the foe defensively, overwhelm him with skill, etc. and no worry about not removing potential hits back as much. It's a matter of degree, however.
It's easy to look at the use-and-forget memorized spell system and think that spellcasters have a limited number of shots and then they become a pile of backup abilities. To an extent that's true. But the non-spellcasters are equally in the same boat of limited uses. Yes, fighters can stab away without running out of attacks. Thieves can sneak as much as they want. But in combat you can't assume you get to attack an unlimited number of times - you're generally going to lose HP during a fight. The spellcasters actually have much more control over the outflow of their resources!
Your spells are more powerful, but you pay for that with limited uses and a much more severe limit on utility magic. If you memorized Fireball instead of Fly, it doesn't matter if you know Fly, you don't have it and that's that. If you need it multiple times you need to have it ready that many times. Spell choices are zero-sum.
This also means the GM is more likely to let crazy stuff work. If you can only get off one abusively interpreted use of a spell each day, that's fine. If it's now an abusive spell you can spam all day, such as in GURPS, you're less likely to get a "yes, I'll allow that." It's precedent plus frequency instead of precedent plus a limited zero-sum resource.
Both are fun games - I'm taking pains to point this out because this isn't some game vs. game argument. They're game vs. game observations about how the games seem to flow in play.