Experience points all the way, by individual character (if a character's not involved in something that grants xp then no xp for that character) - your character gets what it earns via contributing and-or taking risks. I do it this way largely to discourage 'passenger' characters who like to let others take the risk, of which I've seen a few too many over the years.
I also have very slow advancement a la 2e, and a high tolerance for variable character levels* within the party. Xp are never given out for anything outside the fiction e.g. bringing me a case of beer before the session will get you my thanks but won't get your character any extra xp!
* - not recommended for 3e or 4e but fine in 0-1-2-5e.
To avoid having to track lots of ticky-tack stuff, at the end of each mission or adventure I'll give out a "dungeon bonus", whose base value is based on a host of factors (sometimes including my mood!) and then modified per character by the number of adventuring days that character spent in the adventure. (thus if I'd set the base bonus at 6000 xp, a character who was in for the whole 10-day adventure would get 6000 but a character that for some reason was only in for 4 of those days would get 2400) This dungeon bonus in a small way also replaces some of the xp they'd have otherwise got were I using xp-for-gp; the bonus is unrelated to the size of the party treasury. (and some of my players are already greedy enough, thanks!
)
This assumes all the benefits of a new level accrue to the character >
blip!< just like that, much like a typical video game, with no training or rest or anything else required first. I've always had training rules, and even if I didn't I'd still only ever give out xp after an overnight rest (or "long rest" in 5e).
This is one place I've always disagreed with Gygax and other designers: to me, a Goblin that's worth 15 xp at 1st level is still worth 15 xp at 10th level - the difference being that at 10th level you need a
helluva lot more multiples of 15 xp in order to bump, to the point where getting to 11th take wiping out most of the Goblins on the continent and even that might not be enough.