M18 seems to require more micro (stealth) than the M10. I found the M18 to be worse than the M10 in team games (3 v 3)
The M18 in game is bigger than M10 so it's harder to micro, especially in rough terrain. That with better penetration make it a better rushing unit than M18.
Oh and when you try to take on expert AI (no sniper, no AT gun, no doctrine, no early base rush, take no more than half the map, fight with only engineer, rifler, mortar and M10/M18), the M10 perform clearly better against Axis heavy armors spam