They contrast each other very much in terms of their size and build. Familia is tall and massive, whereas Mejia is much smaller. However, on the mound, they are very similar pitchers. They both throw hard. They both expend a lot of energy per pitch and neither have smooth deliveries. Familia almost appears to drop down to throw three quarters, but then comes back over the top with good downward action.
I wouldn't expect either to be groomed as relief pitchers. However, given the inefficiencies in their mechanics, both might be headed there eventually. It could work out well, having these two pitchers - with this level of talent - helping to shorten games for guys like Matt Harvey, Zack Wheeler and Jon Niese in the rotation.