I'm doing Ashtanga with an instructor, and I bet ya, it's hard work!

It's a very dynamic, powerful style, and it helps both with strenght and flexibility. But they sort of come together... you get stronger and more flexible, as opposed to just more flexible or just stronger. By definition, yoga is flexibility, but you can develop strenght too. I'd do it under flexibility, since that seems to be the main benefit.
I did a very brief Hatha course (just 5 weeks), it's similar to Ashtanga, but didn't seem as powerful. I also tried Viniyoga for a bit, and that involved pretty much no strenght. Neither did a yoga DVD I tried a couple of days ago (which was a disappointment, I wanted a good workout!).
I'm in the market for a good Ashtanga DVD and/or mp3 set...