Anything from 0.3 and up would be decent at 50w. Just use 28-24 gauge wire. And don't use old school thank with no airflow and complain why it's hot/Burnt.
if your using premade coils it all depends.
kanger subtank .5 horizontals are too hot at 50
but the vertical ones some people like at 50
while maganus/starre .5 dual vertical coils need about 50 to work well
so it all depend son the build of the coil and the wicking and the airflow