Keep in mind that if you throw a mag extension on a 26 (which it seems a lot of people do), it will make the grip the same size as a 19, completely negating the main advantage the 26 has over the 19: a shorter grip, which makes for easier concealment. On top of that, you still have less rounds than the 19.
If you get the 26, and want a better grip, check out the GAP floorplates. They don't add any length, but people swear by them and how they lock your hand onto the gun. (You still have to tuck your pinky under the magazine, though)
That being said, with a good holster, the right o'clock position, and some forward cant........a G19 is not hard or uncomfortable to conceal, even under a t-shirt. I'd much prefer having 15+1 rounds ready to go with the 19, but that's just my opinion.