If you're visiting from out of town, Twain's is probably your best bet. It's easy to park, there's no cover, the food is pretty good, and there's usually a lot of good musicians there. It's also not a free-for-all with 30 horn players on every tune, and there's lots of other places to go to in the Decatur area. I always liked the Apres Diem session but sometimes no musicians show up. On the plus side, singers almost never show up to Apres Diem
Twain's is 9:00-12:00+ on Tuesdays, Apres Diem is 8:30-11:30 on Wednesday, Churchill Grounds is 9:00-12:00 (I think) on Tuesday. There's another gig on Thursdays at Queen of Sheba that might be a jam session