I've only grown Baptista so I can't really say anything specific about the Amorphas. From what I can see though the biggest difference is that Baptista is an herbaceous
perennial and the Amorpha is a small shrub. I do love my Baptista. I have the traditional blue/purple flowering one and also a yellow flowering one. They are lovely and showy when they bloom and have cool seed pods when they are done blooming. One thing to consider I suppose is that Baptista is a fairly late to emerge plant. This could be a concern if the area is going to get any foot traffic in spring and you don't want to have to worry about where you are stepping. I don't do fall garden cleanup but Baptista seems to be one of those plants that most traces disappear over the winter.