I live here in the western suburbs of Chicago where it is not that hilly, it is mostly flat.
I have been using a 53/39 on the front crank, and a 12-23 cassette on the rear. I purchased a 12-26 for the rear, but haven't put it on yet. I seem to do most of my riding in the 53 ring and in the larger second and third cog in the rear on the 12-23. Also I ride a fixie with 48/17.
Would I better benefit from the 12-23 or 12-26?

If most of your riding is truly in the 53 big ring, you are not getting to the 21-23 cogs anyway. Why would you need a 26?

I live here in the western suburbs of Chicago where it is not that hilly, it is mostly flat....
Would I better benefit from the 12-23 or 12-26?

Are there areas that you ride where 39:23 is not low enough? That'll give you the answer.

Riding in the 53x19 or 21 won't cause any problems if your chain line is OK. If this is the case, a 12-26 cassette might be better, since you will spend more time in the middle of the cassette when in the big ring.

D@mn, western suburbs of Chi-town is "corncob" cassette territory! I'd probably ride a a 12-21 if I live there!

Perhaps a smaller chainring with your existing cassette would be a wiser option. Try switching the 53T for a 52T. That still leaves you with nice close spaed gearing which you'd lose with a 12-25T cassette.

There are some tough little pimples out by the Fox river, south of Elgin, that would bust a gut with a 39X21. They can be done in a 23 standing, or a 25 seated.

I picked up a 12/25 today at Performance. I'll try this cassette out and see how I like it, and if I'm doing to much spinning I'll just keep it for trips to Wisconsin and Galena.