The original post notes a date of Saturday, November 25th. This is the peak travel weekend of the year, yes? IAD is a zoo during the Thanksgiving travel days. This will work both for and against the OP's goals.

Good:

- the chances of a UA IAD->PHX flight being on-time are basically 0.

Bad:

- if you take the number of people who transit the airport on a given Saturday, then double it, then throw in a whole bunch of abnormal, holiday-related flight delays, you can begin picturing just how hectic IAD will be

- if you miss your flight, you have almost no chance of getting home before Monday, since every Sunday flight will be oversold


If you're asking my opinion, I recommend the 7:02pm flight through LAX. It's not direct, and it's a few more hours of travelling, but ensures you get home Saturday night. Either way, best of luck, traveller!