They both have similar functionality but more people using windower and therefore more support for it though questions are answered fairly quickly usually on their forums. Ashita updates/fixes seem to get pushed a little faster and ashitacast xmls are easier to write/understand than the gearswap luas but iirc gearswap can do a little bit more though a lot might not use that full functionality. I tend to prefer ashita partly because a long time ago windower stopped working for me and I like the multiboxing plugin better. And no spellcast wont work with gearswap.