I haven't actually used the 600 medals yet. I've been strongly desiring either Easter Cactus or a second -20% debuffer next to Acacia (I have Shinshun Ivy, but she only debuffs 2), but I haven't made up my mind though. My roster is already capable of handling the core stage of Nidhogg, so I think I won't be rushing to pick anyone for now.
I will likely leave my options open and see what I get from the pre-reg ticket.