you can't exactly hook up grids, louvers, baffles, flags, gobos, etc on an umbrella. But, an umbrella is best for smaller spaces.
I use 2 60" softboxes, left to right @ 45deg angles, and a large umbrella over my head/camera for fill sometimes. That provides a really soft light for massive fill. And, it doesn't make me have to bounce off of ceilings/walls.
Of course, you could always shoot through an umbrella and get a softer light...
But, the Westcott Halos are really cool. Kinda like a hybrid between a softbox and an umbrella, AND, they use less space than both!
Plus, an umbrella doesn't work for a hairlight/rim light.
My next purchase...a 7' Octabank softbox. Great because the monolight goes in it. So an Elinchrom Style 1200 is coming with it!