Same situation here. I have had the 12.9 first gen since day 1 and got used to the size pretty easily but I wanted to give the 10.5 a try. There are things about both that I like though, now making my decision all the more difficult on which one to keep. In the back of my mind, the specs of the 10.5 tell me that it is a powerful beast and can handle anything coming down the line but I also wonder, with the first gen 12.9 being relatively new tech, it can be that far behind. I have the ASK for my 12.9 and I have moved away from using it to using the software keyboard which is really nice on the larger screen. You do notice a difference by not having to bounce around like on the smaller keyboard, but I guess the upcoming version of iOS adds more functionality to the keyboard so this may be a non issue anyway.
My work is about an equal 50/50 mix of mobile and desktop work as well as watching movies and reading news in the downtime. Such a tough decision with two fantastic devices to choose from!