For efficient lighting, i guess nothing beats a direct front on flash. A reflective umbrella will be more efficient than the umbrella softbox and softbox.
I am currently using the umbrella softbox (38inch one if i am not wrong) from artworkfoto and it is easy to setup, fits into the umbrella holder well. However, your flash distance to the umbrella reflective material means u need to set your flash to the widest possible setting so that your light is well distributed (unless u using those bare-bulb flash). I like this umbrella setup coz it has a widest possible spread, has a close to round shape (for pleasant catchlights in portrait shots) and easy to set up. For narrower spread, I will zoom in my flash to max setting and even tape a black cloth/paper over the areas that i want to tape.
I was considering the softbox then but it costs almost double, a bit harder to set up and some softboxes dun have the diffuser panel to diffuse the light, so u may end up with hotspots in the center. However most softboxes come with diffuser panel on the interior and also add-on grids for better control.
A shoot-thru umbrella throws some light backwards too and may introduce extra light to your subject if nearby walls/objects reflect them back... this may cause problem on control of light if you want to light a particular area. Flagging/Goboing a curved surface is also a challenge. But a shoot-thru umbrella is definitely good for some shots where u want 'harsher' lights on the subjects and also do not mind or even prefer light to bounce off the walls to fill up the shadows...
So i guess, it really depends on how u want to light your shot.

U may end up buying all the different types of light modifiers...