I have a pair of the Pro Elite powerblocks.
Here's a reasonable side-by-side comparison:
Power blocks: up to 130lbs/hand
Bowflew: up to 52.5lbs/hand.
Powerblocks: 10 year warranty
Bowflex: 1 yr labor, 2 yr tumblers, 5 yr plates.
Powerblocks: metal plates.
Bowflex: plastic covered.
The one drawback on the powerblocks is the the increment skips 5lbs from a weight that ends in 0 to one the ends in 5, so:
5, 7.5, 10, 15, 17.5, 20, 25, etc, etc.
To get a intermediate from 0 to 5, you'll need to use plate mates.
The SelectTechs has a single width that some people complain about when doing exercises tight on space. The powerblocks get narrower as the weight goes down.
Still, I love my powerblocks and consider them the single most important addition to the home gym.