If they are all in your budget, get the most expensive one, chances are you can get the really cheap 40/2.8 for father's day or birthday. (I love mine, and people don't feel bothered by "that guy with the big lens")
The 50-135 will get you the most use and is the hardest to get at any other given point in time but now.
The problem with the 40/2.8 is also that you will go "Pancakes are fun, now i want the 21/3.2 and 70/2.4 for a small prime + body pack".
And like you said, the 50-135/2.8 will complement the Sigma 24-70/2.8 for events.
You already made up your mind, me thinks, you just wanted to hear 50-135/2.8 a few times....