Frontier or United?
 

I have a trip from Chicago to Denver coming up. UA and F9 both have an early morning flight on an A319. Both are around the same price. MDW is slightly more convenient than ORD, but the UA flight is slightly earlier...
I have low-level elite status on UA, and have never flown F9 before.

Should I give Frontier a shot? Is there any food service to speak of? How does the Denver terminal compare to UAs?

I'd suggest giving Frontier a try. There will be no food service, however. The 319's are comfortable and you may have Direct TV to watch ($5 charge and I don't believe all the airbuses have been retrofitted yet). Frontier uses concourse "A" in Denver, which means you can walk to the terminal and don't have to use the train. I like their FF program, Early Returns. Enroll online and receive 1500 miles to start. Free domestic ticket at 15,000 miles. They also have a new credit card partner. 7500 miles to sign up for that. Most of the employees you will encounter will be friendly. Did I cover everything?

So did you make a decision?

I have been flying Frontier to LAX occassionally in the past month. On both the 7am and 8:50am flights there is beverage service and also a bagel (Plain or Cin/Raisin) with cream cheese.

The concourses are basically identical, and as mentioned before you can walk to the terminal instead of taking the train if you would like. I believe that DirectTV is available on all A319's with the $5 fee, it is free while the plane is on the ground and during takeoff. If you do not pay for DirectTV there is a map display that rotates with some ads occassionally on the screen.

I have been happy with Frontier the times that I have taken them, and it seems that their employees seem to care.