The longer we played RM the simpler our Exp system got. We started with the normal system and made minor modifications, then we moved to a mission/goal based one, then we moved to a session based amount (at the end), then I finally just decided when I run it's simply when I tell you to level (which lets me control the pace of the campaign).
That is basically what I did (over the course of twenty-five or so years).
That said, we only play RM* nowadays as a day quest (we do four day quests a year, and what the are really depends; they could be RM, D&D 3.x or Judge Dredd D20 these days). The only exception was about ten years ago, where I briefly ran a campaign in the evening (basically by stapling together about four or my previous day quests that this set of player had never played), but I am not even sure if they levelled up at any point there or (it would have been at a mandated point if the had). The quests are thus generally not contiguous, aside from the odd two-or-three parter, with in-universe time gaps between.
Further, the only other RM party we have played (until a week on Saturday) was one that had been kicking around for knocking onto twenty years and I have stopped advanced altogether on account of it becoming more or less impossible to present a combat challenge. (Despite them only being level 12.) But those games are as close to sandbox as we've ever gotten (in the sense that sometimes I would attempt to squeeze blood out of a stone and ask the players what their characters wanted to do next game) and was more about faffing around with those characters than advancement. (And also, of course, with only about six-eight hours or so for a day-quest, you aren't playing them long enough to have to worry about advancement.) So it wasn't really so much about mechanical character advancement.
(Having said that, neither I nor the other DMs track XP for day quest even when using something like D&D, though.)
For our weekly sessions, we tend to play D&D (again 3.x if I'm running (Paizo adventure paths nowadays), else 4E sometimes) and there, I do track the XP. And of course, D&D 3.x (and 4E to a lesser extent, though I'm not fond of the system) is a much more character-mechanics/character-build/character-progession based system, so scratches that itch rather better. (After all, in RM, you only have skills, so aside from the very few spellcasters, you only get better at what you do, not find new things to do most of the time.)
We are about to start our first new RM/SM party for ten years shortly, where they party have been set at level 6. (And they are Liches and part of one of the galaxy most high-tech powers; the fact this is a DOWNGRADE tells you something about the previous RM party...) I am not planning to advanced them after the first two-parter.
One thing I've found over the years, is that a party has a "life." At the end of which is the point where either the players are bored with the party or the DM can no longer sensibly concoct challenges for them. (4E went as far as actually codifying this by an actual level cap.) In D&D 3.x it's around 20th; we've run one party from 1st to low-Epic, and by the end, it was taking a disproportionate amount of time to sort the monsters for me. So, these days, with a D&D party, I work along the lines of "we'll play this party through the adventure path, and that will probably be it." With our RM parties and day-quest D&D parties, keeping the advancement rate down help extend the longevity of the party. (Especially in D&D, where a rate of one level per day quest which of one the other DMs does, I felt was far too fast, as my character didn't really get much chance to play with his new toys before he got his next lot.) RM is also bad enough to try and have meaningful combats in anyway (given players have tendency to open-ended splat EVERY boss monster), compounding the issue by inflating their numbers - especially in sci-fi, on high-end weapons where you can easily reach K-M crits and hundreds of hit damage - does not help.
Thus, out day-quests are a different sort of game to our weekly ones, and the criterion for advancement (and XP) are different.
*I say RM, I really mean RM/SM, since we haven't played an actual fantasy RM (as opposed to RM/SM sci-fantasy) game for... Well, the last one we played - and the only one fully on PC - dates back to 1998.