I own the 28" Westcott Apollo and a 15" collapsible softbox (like the Lastolite you linked to). I can confirm that the 15" softbox is much, much easier to work with. With that in mind, I'd suggest a 24" Lastoliteebay clone. It's about $60 with shipping and includes the speedlite mount. I don't actually own the 24" version, but I'm seriously considering purchasing one myself. The 15" version I have is an Alzo branded softbox--and the quality of it is quite good (although it's a bit small for anything outside of head & shoulders shots).