The reality is that for a meager $3.25 to get to the airport, this mode of transportation is a steal.
I've used the UP Express and will say that its nice to have a shorter commute sometimes when trying to get to the airport for a certain time, but if you aren't on a timeline or plan ahead well enough to give yourself the extra 20-30 minutes of buffer time, this is just as good. Coming back into the city from a trip always resort to the Rocket over the UP.
It's a quick ride to Kipling which then you transfer to the subway and head downtown as needed.
The fact is that the last subway train leaves Kipling at 1:31am each night. The UP Expresses last train is also at 1:30am. So while you need an additional 15-20 minutes to take the bus from the airport to Kipling, in the grand scheme of things these hour differences are so tiny that if you can navigate it, I'll still put my backing into the 192 Airport Rocket as my first choice.