If you can find a range where you can rent pistols, try to shoot both and see how they feel. That's the main difference. When I was shopping for a carry gun, I looked at a 229 and a 239. Strangely enough, I shot better with the 239. Had I looked only at technical specs, though, I would have chosen the 229.
Between the 226 and 229, the latter is considered to be more concealable. Even though the widths of these two models are listed as the same (1.50"), the 229 slide is noticeably thicker. It's a shame that SIG didn't slim it down and maybe trim a bit more overall length, to make these models more distinct. The 226 is considered a combat pistol and the 229 more of a police/carry pistol, but not strictly.
They're all a bit top heavy, but great for shooting--you kind of want that heft for .40. I have a 239 in .40 and never regretted it. People who carry them only speak highly of this one, even though single stack is kind of old school

Of course I want another SIG, and I'm going through the same agonizing that you are--226 or 229? Can't decide!