I'd love to see a level 10 fighter vs a level 17 fighter all stated out. I want to get an idea of how much more powerful they are.
Assuming identical ability scores. At level 17, the F17 would likely have weapons and armor +1 or +2 better than F10.
The differences would be:
F17 has 21 more HP than F10 (about a 35% increase or so assumign CON 16, which is +2 HP in AD&D).
F17 has +8 better attack rolls (not including the
very likely +1 or 2 more from magic items of superior quality) than F10
F17 has maybe +1 to +3 better damage, but only if superior magical weapons.
F17
might have better weapon damage if using a weapon they are specialized in that F10 is not.
shrug
F17 has an extra weapon attack every other round than F10.
F17 has +5 better saving throws than F10.
If we give them typical armor, shields, and longswords, you would likely have:
F17:
AC: -4 (plate +2, shield +3, dex -1 to AC)
HP: 91 (CON 16)
Attack: +8 (double-specialization +3, STR +1, longsword +4) with THAC0: 4.
DMG: 1d8 + 8
F10:
AC: -2 (plate +1, shield +2, dex -1 to AC)
HP: 67 (CON 16)
Attack: +6 (double-specialization +3, STR +1, longsword +2) with THAC0: 12.
DMG: 1d8 + 6
So, the F17 would need a 6 (THAC0 4 - -2 AC), but gets a +8, so can't miss since the minimum modified roll would be 9. IIRC, a nat 1 automatically missing was not a rule in AD&D, but I could be wrong since everyone played it like that anyway...
With average damage of 12.5 (4.5 + 8), the F17 would kill the F10 in about 3 rounds or 6 attacks.
F10 needs a 16 (THAC0 12 - -4 AC), with a +6 means a 10 or higher on the d20. So, roughly 50%. With average damage of 10.5, F10 would do about 21-31 points of damage before being killed. That is less than 1/3 of F17's HP.
Obviously the biggest difference is the +8 more to attack F17 has over F10, and the +5 more to all saves would help as well (though not likely in a direct battle between the two...).
Damage isn't too much different, and the one extra attack every two rounds is nice but not huge. Take away the superior magic armor and weapons, and F10 fairs slightly better, but it really is no contest.