I don't know if you're still looking for a reply, but here goes:
"On Touch" can be difficult with a weapon, because it basically requires that you are right up against them when it strikes. Longswords have enough range that it is possible to be close enough to hit the target, but too far away for the "on touch" effect to hit them. For this reason, I think it is best to use "On Target" enchants for weapons, even if they are a bit more expensive (as far as how much charge they use per strike). Adding an area is only required if you want to be able to take on multiple enemies at once and have the magic hit all of them. That, of course, uses charges faster, so it may be preferable to have 0 area.
a.k.a. "planty" (thanks Stargelman)
official fan fiction verbose commentator
master of fungal death magic