2 words: Arthur PF.
Oh right, and I forgot Hercules is normal soul points as well. Hell, even more important than Arthur, considering she has one of the best relics in the game.
Joan will save your ass big time against hard hitting nukes, and boy are there a ton of time incoming. You're giving her less credit than she's due my m8.
Can't really speak for thunder specifically, but Joan over Arthur IMO, and I would get Herc before Joan if you already have defensive kamis. I personally think Andromeda is massively underrated depending on team comps, but that was mainly for water so that might be why. Honestly though as long as you can clear most normal content, or you're looking to buy the one soul that your comp would use for hard content, which to buy next isn't that big of a deal as you'll be able to buy them all before too long (especially with advent battles having 600 soul points if you really want it quick).