I have no idea whether it is accurate, but I always used +2 to-hit = +1 to-dam as my guideline.

I just did some math. (The displayed hit probability is hopelessly wrong for the monster info screen.) It depends a lot on your melee skills. Let
M = melee skill = base + 3*to_h bonus.
AC = monster AC
Then hit probability is
.12 + (1-.12-.05)* (M - AC*2/3)/M.

For M = 120 AC = 150 (roughly a dracolich) you will be hitting
26% - Base
40% - With a +10 accuracy ring (or buffed with !Hero alone)
57% - Buffed with !Hero and !Berserk
62% - With all 3 bonuses

So without buffing (Note: not recommended against high AC), your damage improves by .14/.25 = 56%. With heavy buffing, it increase by 9%

For a decent midgame damage output of 50 per blow, that corresponds to 26 and 5 damage per blow, respectively. Of course, if you're using Sting or Harradekket, the numbers are much lower.

TL;DR:
For good melee compared to level appropriate AC, the value is about 2:1 to_d:to_h
For poor melee, always choose to_h.

The other thing to keep in mind is that these comparisons are important *only* when facing a serious threat. You simply shouldn't care against lesser opponents.

With one exception: mage. To conserve mana, you need to fight everything except for the most dangerous monsters. The optimal mage melee weapon is thus a MoD of Gondolin--ie, slay everything you can actually hit. (Of course, you may need a stat stick here.)

Do such mages ever choose between a ring of damage and a ring of slaying? I think I always use different rings by the time that sort of combat is feasible.

Yeah, mage physical stats are so miserable that they'll almost certainly prefer =STR or =CON over either of those.

Eddie--if you're a high elf mage using something like Thunderfist(CON, STR) or Deathwreaker (STR), I can see adding a ring of slaying as an option. An extra 40 melee makes a big difference against high-AC targets.
Edit: one reason to point this out is that mage hit probability is *significantly* overstated by the 4.0.5 monster knowledge. (Nick is or has fixed it in the rev branch.) If you are considering a fighter mage, focusing on this really helps (along with al three buffs, despite the nerfed Tenser's mages get.) Of course with archery you get a big bonus from additive hit chance of both bow and arrows, so long as you are shooting from relatively nearby.

If you are using it to save mana, you might want to wear a ring of int. Or con or speed, as death reduces your mana precipitously :). In particular, a ring of str+5 generally gives you +5 to hit and +5 dam in addition to possibly another half blow or more. It's really hard for a ring of slaying to match that, and if they are even close sustain str is nothing to sneeze at. I've played a lot of everything, including mages, and I really cannot remember a mage giving slaying or damage serious consideration, ever, except maybe early on before I found a decent stat ring.

Mind you, my memories and experience are mostly for 3.0 with my version of blows code, but I've played some 3.1+, and I think the principles should apply pretty well to 4.0.