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. 


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


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.