I have a Pac bag and a Push The Envelope bag... both are awesome, but I like using the Push bag a little more. The Pac bag strap design makes it really hard to open the bag without readjusting the main strap length all the time.. the Push bag has the strap mounted on the back of the bag instead of the sides, so it opens all the way even when the straps are tight.

The other thing I prefer on the Push bag is that it has multiple interior compartments.. they are all flat so you can still use the entire space for big stuff, but you can also organize. Only the really high end Pac bags have that..

Construction on both are top notch, I'd recommend trying to see them in person before deciding.