If you like Firefly, there's a better-than-even chance you'll like Buffy & Angel as well.....especially the latter. I know those of a more SF persuasion tend to have an instinctual aversion to these two for some reason, but if you give them a fair shot I think you'll find they're pretty good. They have all the same strengths as Firefly, except with monsters and magic rather than spaceships and horses.
Now, personally I watched Angel first, and then went back to Buffy. That's doable. However, you'll probably get more out of both of them if you begin with Buffy and then transition to Angel when that show spins off.