Another reason to try El Ideas, given that my above comments didn't seem to sway you - Kevin McMullen just made Eater's 'final 50' list of "Young Guns".
I'd worried about him last time I dined at El Ideas as he was cutting Iberico ham (I think, might have been a different fancy and delicious ham) paper thin on a meat slicer that was well overdue for sharpening, if not outright replacement. He's a rock star and represents another great thing about El Ideas - Chef Foss is a generous collaborator who gives similar free reign to young chefs (like Kevin and formerly Michael DeStefano) as he does/did to 'senior' chefs (like Andrew Brochu).
I highly recommend getting in while Kevin has all of his fingers! He's a star!
I would absolutely go to El Ideas. Here's my reasoning:
1) As the Beard nominations attest, Carlson is in top form and even reluctant foodies in Chicago are starting to acknowledge that Schwa has gotten some of its pre-hiatus swagger back. My suggestion that you try El Ideas is in no way a dig at Schwa. If you were asking for advice on which to pick if you could only go to one, I'd tell you to go to Schwa.
2) The fact that you had a good experience at Schwa (although, how could you not?) indicates that you are open to chefs that push the envelope conceptually. Schwa tends to be a little more avant garde in its food than El Ideas, but Chef Foss isn't shy about trying adventurous flavor, ingredient, preparation and presentation combinations and bringing smiles and excitement to his diners' faces. You will have a great meal and at least a few dishes that will make you think in new ways about food.
3) Where El Ideas really shines is that it has taken the entire concept of a restaurant and redefined it. If you liked how informal Schwa was with Carlson serving tables and diners welcome to walk through the kitchen, El Ideas will rip your skull open. El Ideas is not a "restaurant", it is not an "underground restaurant", it is not a "dinner party at your friend's home" (unless your friend happens to be a leading edge chef) - it is something completely new. Go to El Ideas for the experience.
Now for some unsolicited advice: Since you come to Chicago so frequently, and enjoy fine dining BYOs, I'd also strongly encourage you to try two other "fine dining" BYOs at the top of my Chicago restaurant list: Goosefoot and Bonsoiree. I ate at Goosefoot for the first time two weeks ago and was blown away. Less adventurous than Schwa or El Ideas, but unbelievable in delivery of culinary delights. Chris Nugent is killing it. Bonsoiree occupies some middle ground in this grouping and deserves a spot. It is excellent. Finally, if you're looking for a great, mid-priced BYO, try Ruxbin on one of your visits. It's just down the road from Schwa and will not disappoint.