With regards to XP distribution and loot, a battle only gives you stuff for enemies you either kill or talk to. That means if start the battle with four shadows, kill one, then knock down the other three and negotiate with a shadow to end the battle, you only get the XP/benefits from the enemy you kill and the one you talk to and you lose whatever you would have gotten from the other two. So doing an all out attack is the best option for maximizing XP and loot drops.
As for your other options, that boils down to the kind of resources you want and the kind of confidant ranks you have. The short answer is, if you're short on money, then you would want to try and negotiate for money instead of items. If you want healing items, you would ask for items, and so forth.
But confidant perks and day activities can complicate things slightly since you can buff certain negotiation tactics. For example. you may be better off asking for money since you boosted your money output and earned the option to repeatedly ask for more money from hanging out with somebody. You'll get a better handle on what kind of perks are available to you as you play the game and learn about what options the city has to offer.
Also, keep in mind that a holdup guarantees an end to the fight if you just talk down one shadow and an all out attack isn't guaranteed to kill everything, so it may be worth it to lose the XP just to end the fight and avoid damage to the party.
And finally, if you kill with an All Out Attack, you get a freaking sweet splash screen. And in a game like P5, I consider that a reward in its own right.