One thing I never liked in AD&D was that crossbows are the much-poorer cousin to bows. Maybe Gary Gygax was really swayed by those accounts of Crecy, but it seems odd that crossows are basically slower, weaker bows. So much so that no one ever took them until I made some house rules.
Here they are:
Heavy Crossbow: 2-7 vs. S-M, 1-6 vs. L, ROF 1
Light Crossbow: 2-5 vs. S-M, 1-4 vs. L, ROF 2
Range, cost, etc. are the same.
The heavy crossbow is now a once-a-round weapon that does superior damage on that single hit to the bow. Still not as good as a bow, since 2 x 1-6 is better than 1 x 2-7, but it's not a completely inferior choice. Items like the Crossbow of Speed move this up to ROF 2.
I used to run it with ROF 1 for both of them, but I find the light crossbow needs some help to be a valid choice. And "Crossbow" is one proficiency in my games, such that proficiency matters.