Grayhawk said:
Specifically when it comes to spellcaster's chance of hitting monsters. Do the two methods scale about the same or is one notably better?
It's variable.
What you are proposing works in favor of the spellcaster against low AC opponents, but often against them against high AC opponents. The two don't necessarily balance out.
You said you have a low-power, low-magic game in mind, so I used CR 8 creatures with nothing more than first level spells and reasonable low-power stats. Math-fu follows:
Mind Flayer, CR 8
AC 15, or Touch AC 13
(from Monster Manual)
Mohrg, CR 8
AC 23, or Touch AC 14
(from Monster Manual)
Fighter 8, CR 8
AC 25 (20 without expertise), or Touch AC 17 (12 without expertise)
(Dex +1, Dodge Feat +1, Combat Expertise +5, Full Plate +8)
Dire Tiger, CR 8
AC 17, or Touch AC 11
(from Monster Manual)
Wizard 8, CR 8
AC 20, or Touch AC 12, or AC 12 for both without spells.
(Dex +2, Mage Armor +4, Shield +4)
Attacking them we have a Wiz or Sor 8, with 14 Dexterity (+2 modifier) and 18 in primary spellcasting stat for a +4 modifier
So...
Rules as written:
Ranged touch spell: d20 + Dex + BAB = d20 + 2 + 4 = 7-26
Chance to hit AC 13 (mind flayer): 70%
Chance to hit AC 14 (mohrg): 65%
Chance to hit AC 17 (fighter): 50%
Chance to hit AC 12 (fighter without expertise): 75%
Chance to hit AC 11 (dire tiger): 80%
Chance to hit AC 12 (wizard with spells): 75%
Chance to hit AC 12 (wizard without spells): 75%
As you propose:
Ranged touch spell: d20 + Int/Cha + Caster Lvl = d20 + 4 + 8 = 13-32
Chance to hit AC 15 (mind flayer): 90%
Chance to hit AC 23 (mohrg): 50%
Chance to hit AC 25 (fighter): 40%
Chance to hit AC 20 (fighter without expertise): 65%
Chance to hit AC 17 (dire tiger): 80%
Chance to hit AC 20 (wizard with spells): 65%
Chance to hit AC 12 (wizard without spells): 95% (1 on d20 always miss)
Look at what happens to the low AC opponents. Ouch. High AC things get a little harder to hit.
*shrugs*
I'm not going to tell you how to design your game, but my humble opinion is that Touch AC isn't any more complicated than anything you have proposed, and what you are proposing introduces some balance issues. I do not know what could replace Touch AC that scales well and does not introduce balance issues. Anyway, if you think the effort to get it to work is worth it and you like the results, good luck to you and happy gaming.