Ok, low level data, but that's all I got.
Tested skillup rates between light thrust and Skewer. Using a Weathered Spear, rank 6 lancer, physical level 9. Fought Star Marmots just south of camp Black Brush since there's a nice concentration there. All mobs conned blue. After hitting rank 7 lancer I stopped. Only recorded hits, since I'm assuming you can't skillup on a miss. All skillups were treated equally, the best were about ~50 points for both Light Thrust and Skewer, the worst was a 4 point Skewer, but they all counted as simply a skillup.
The assumption was that rates would be better on skewer, due to it doing higher damage and being usable less often. However, the data suggested otherwise. Out of 100 landed Light Thrusts, 27 produced a skillup (27% chance). Out of 34 landed Skewers, 9 produced a skillup (26.47% chance). From this, I make the assumption that skillup rates between the two are identical.