I like the Bowflex's quick-change system but they have a rep for breaking or getting stuck. I went with Powerblocks because of that. They seem unbreakable and are made in good ol' USA but the quick-change system when using the "tubes" for 2 1/2 and 5 lbs intervals can slow you down if you're doing a fast-paced program like P90X. I always fall behind in the shoulder/arms workout. If you're switching 10 lbs increments then it's quick. Also, when you go with a single tube (2 1/2 lbs), the weights feel off balanced, but at least that option is there (Bowflex only has 2 1/2 increments in lower weighs). The cage design mentioned can be annoying and snag your clothes but you can get used to it. Finally, Powerblocks can be expanded to something like 120 lbs while Bowflexs stay at whatever max weight you bought (90 lbs max).

So, in a nutshel:
Bowflex Select Tech: Quicker change, more natural/balanced feel, less bulky.
Powerblocks: More durable, smaller increments, expandable, higher max weight.

Hope that helps.