You already own a G26, so sticking with the caliber will be the best idea. The G19 is not that much different from the G26, whereas the G17 will feel and shoot different. Since muscle memory is developed by practice, and it is what your instincts will do in a no-warning-high-stress-situation, get a G19.
You can also add magazine buttplate extensions to your G26 that add a pinky rest and change the feel of the gun entirely, or you can get +2 extensions that do the same while adding to your magazine capacity. I have and love the G26, now that I have the extensions on them!
Happy shooting!