I can't answer to Earth wands, being fully against them even as an Earth caster for reasons stated in other threads, but I'm a very math-loving person, so I'll fire up my calculator and start jotting down scaling ideas.
At least to start, I'm going in operating under the assumption that a) Proficiencies/Backstabs should scale in such a way that a Fighter or Rogue has more damage per swing than a Scholar would have damage per wand charge at the same amount of XP spent in "class" skills, b) the Wand charge scaling (1 charge per 3 XP, 1 damage per 50 XP) is remaining as-is, c) Weapon Skills (including Shield, Florentine, and Two Weapons) do not contribute to scaling, and d) you don't stray too much outside of "pure" Fighter/Rogue/Scholar skills (no more than about 10% of your XP is spent on other skills and prerequisites).
At 105 XP (level 10), you've spent 90 XP on skills within your "class".
Wands have 30 charges, and do 2 damage per charge.
20 XP per Prof/BS: With Profs you're swinging for 6 with OHE (9 with 2-Handed weapons), with Backstabs you're swinging for 10 from behind with OHE.
30 XP per Prof/BS: With Profs you're swinging for 5 with OHE (7 with 2-Handed weapons), with Backstabs you're swinging for 8 from behind with OHE.
40 XP per Prof/BS: With Profs you're swinging for 4 with OHE (6 with 2-Handed weapons), with Backstabs you're swinging for 6 from behind with OHE.
At 205 XP (level 20), you've spent 180 XP on skills within your "class".
Wands have 60 charges, and do 4 damage per charge.
20 XP per Prof/BS: With Profs you're swinging for 11 with OHE (15 with 2-Handed weapons), with Backstabs you're swinging for 20 from behind with OHE.
30 XP per Prof/BS: With Profs you're swinging for 8 with OHE (11 with 2-Handed weapons), with Backstabs you're swinging for 14 from behind with OHE.
40 XP per Prof/BS: With Profs you're swinging for 6 with OHE (9 with 2-Handed weapons), with Backstabs you're swinging for 10 from behind with OHE.
At 305 XP (level 30), you've spent 270 XP on skills within your "class".
Wands have 90 charges, and do 6 damage per charge.
20 XP per Prof/BS: With Profs you're swinging for 15 with OHE (19 with 2-Handed weapons), with Backstabs you're swinging for 28 from behind with OHE.
30 XP per Prof/BS: With Profs you're swinging for 11 with OHE (15 with 2-Handed weapons), with Backstabs you're swinging for 20 from behind with OHE.
40 XP per Prof/BS: With Profs you're swinging for 8 with OHE (11 with 2-Handed weapons), with Backstabs you're swinging for 14 from behind with OHE.
The most reasonable scaling seems to be in the realm of 30 XP per Prof/Backstab equivalent; too much lower and you see damage numbers skyrocket very quickly, too much higher and it doesn't produce a significant advantage over Wands.
Scouts don't scale up in damage as well as one might hope; a Scout with XP evenly split between Fighter and Rogue skills falls behind in damage as they go up in levels. Using the 30 XP scaling above, a level 20 Scout is swinging for 5 from the front with OHE and 11 from behind; at level 30, they swing for 6 from the front with OHE and 14 from behind. They have cheap-ish access to skills from both sides, however, so they're likely to have more utility and a greater variety of abilities than their "pure" counterparts.
Adepts and Spellswords who split their XP evenly seem to fare pretty well using the 30 XP scaling, I think: Spellswords are swinging 5 at level 20 and 6 at level 30 with an even split, while Adepts swing 8 from behind at level 20 and 10 from behind at level 30 with an even split, and both will still have their spells (and Wands, with 45 charges dealing 3 damage each).
TL;DR:
30 XP per Prof/BS-equivalent damage increase seems to be the most reasonable choice, offering more damage faster than a Wand while not skyrocketing off to ridiculous heights until a character is very very high level.