SED is best for Elite Guard IMO, does more damage to bosses letting you get last hits and and such, and if it's high enough you can solo girls without puddings. As far as the 10 dollars goes I don't think matters where you put it. 10$ isn't really going to make a huge difference, personally I'd just wait and put money away for the game until I go all in on an event and try to get the slayer girls to get their SED modifier bonus.
P.S. Also if you don't have the means to boost the slayer girls up to 100 they won't be as useful, I believe their SED modifier increases with level