I'm exactly in the same situation and sentiment, but still trying to decide between the 12.9" and the new 10.5" ipad Pro.
Les, I've used a 10" Samsung for the last two years. My wife has the larger version and I've used it frequently. Get the larger version. Especially for photo editing, you need all the screen space you can get. Precise pointing with your finger is much easier with the larger screen.
The only downside is weight. The larger version weighs 1.5lbs. It's not difficult to hold, especially if it's in a cover of some kind, but holding it with one hand and using the interface with the other (while standing, for example) can get tiring. Other than that, it's a wonderful instrument. The pen action is nothing short of magical. By all means, get the pen.
The larger-version screen is nearly exactly the same size as a standard magazine page. Clever, them Apple people.