I live in Phoenix as well and have commuted a lot over the years.  Saddle bags are the way to go if you can.  I have a set of Ortlieb.  In one bag I can fit all my work stuff and climbing gym gear if needed. 

Pros:

Weight is off of you.  Your back can breath so its cooler.  Bages are massive. 

Cons:

They suck to carry around.  You need to secure all the straps every time you ride.  Getting a strap stuck in your rear tire is a pain. 

Having said all of this, this for pure commuting.  I'm a mountain biker at heart with trials tendencies.  More often than not I'm riding with a backpack because I link some trail riding in.  Also I can't hope a bike with saddles bags.  But... when I do have my commuter bike with saddle bags it feels like I'm playing on easy mode.