By relying on ghost, I rather encourage you to make tons of potions out, which is much better than spending on a summon which has time limitation. You can creates hundreds of potions with the potion trick within 5 to 10 minutes. And with some ranks on potion efficiency, you can close to fully heal your hp with just use of potion.
As for summons, I will only encourage getting demon if you want, as it seems more useful to me.
