I prefer just taking ability scores out of success chances altogether (attack bonus, skill bonus, etc.). They matter too much there, and make the game harder to balance, unless scores are radically curtailed in ways that I don't think will happen. That's my #1 choice.
However, for a compromise position for play concentrated in the lower levels--especially for certain playstyles, I can see a place for letting the abiity score bonuses run loose, but do not let them stack with anything else. Then include a module to let them stack somewhat, but with limits.
So in the base version, an 18 Str gives you +4 to hit and damage with all melee weapons. When your attack bonus from skills, feats, etc. is lower than +4, you get +4. When your attack bonus from those hits +4 or higher, you use that "skill" bonus from now on. You continue to get the +4 to damage, as this does stack with any damage bonuses. In a system with a smaller range of attack bonuses, that could easily be relevant for some stats up until around 10th level or so.
The more complicated, simulation-oriented version would work a lot like the old Basic potion of heroism. A zero level commoner drinking it got boosted to 4th level fighter, while a 10th level fighter got nothing. In between, it was graduated. So you'd compare success boost from ability scores to success boost from everything else, and get less and less of the ability score boost as you go--comparing just mod to mod. By the time you hit +10 in skill (and related), you might get a maximum of +1 out of ability score.