Hard to comment without knowing what you're shooting but,
here's my $0.02 (at todays inflation rates, that's about all it's worth)
The Omni for portability and set up speed...but the soft wrap of softbox light is hard to beat...
You have to weigh the options...set up speed VS quality of light...not saying that the Omni cannot produce great light, but I LOVE softbox light...and if time permits I ALWAYS use it....
The softbox pictured, I wouldnt use it (caveat: I don't nor ever have owned one, so am speaking from a personal perception of small softboxes being simply oversized clunky Omni's)...I would just go Omni if that was my only other option.
If I was going to set up a softbox, I have three 60cm x 60 cm that I use with 580EXII's when I have time to set them up...when I dont, the Omni's go on...
Weather is also a consideration, on a windy day, when those big 2ft x 2ft softies are getting blown all over the place, well, either I haul some heavy sandbags along to keep the stands in place, or, use Omni's
Options are wonderful though, and you should consider having both and knowing both tools are in your arsenal....
Here in Vancouver I was able to locate Omni's on craiglist for $10 each and 3 x 60x60cm softboxes (including speedlight brackets) for $55 each...all told, $225 for a complete solution of softboxes and OmniBounces. (I have 5 x 580EXII and 2 x 430EXII's)
And speaking of stands...no more spending $150 and up for "photography lighting stands" I now shop at the local Music Store and buy high quality boom microphone stands for about $50-60 each
http://www.k-m.de/en/products/category/starline-32/?no_cache=1
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Whatta Hobby this Photography stuff is! Expensive!