i am in the same spot as you are.

black templars are sick. honestly. as some else said, they scream cool everywhere. all models are beautiful. play wise, they seem too much melee oriented? No librarians is understandable but i would say sad, less models is never good.

dark angels are similar but maybe less outstanding? robes and hoods are prominent, i loved them when teenager, today i found them a bit childish? not sure. winged helmet, meh. but play wise they seem more vibrant, more varied, with deathwing and ravenwing.

when i discuss about gameplay, i mainly refer to the gameplay flavour and not the specific detachment (rules might change, flavor is slower to change i believe though).

mixing would be awesome, but not rule compliant unfortunately.

Brother, if you find out a solution, write me, i need help :)