Why is viewing in standing position in portrait mode also difficult on the 12.9" ? Why would that be difficult ??
I find it awkward using iPads while standing up. Even the 11" feels heavy/offbalanced when holding it in 1 hand and navigating with the other but that's just me, others seem to have no issue using them this way. I've never been comfortable using the tablet in any way which requires me holding it up. I always use the keyboard cover as a stand whether on my lap or desk.
Keep in mind the larger the device the further out the center of gravity gets when holding it with 1 hand, so even something the same weight but larger in side feels heavier when not held in the center.
I'd suggest going to the Apple store as soon as it opens while it's still fairly quiet, find a 12.9" that has the longest security cable or ask them if they can unlock one of them so you can try holding it the way you see yourself using it at home, or even sitting down with it at the store to get a better idea. They've unlocked ipads for me during quiet time so I could walk around with it.
... or do what I did, buy both, use them side by side for a few days and return the one you like less
If I could I'd have kept both.
Side Note:
While I mostly like the new keyboard cover I miss having the keyboard and back cover as 2 separate items. I'd sometimes detach the keyboard and having a cover on the back not only protected it from scratches/dents but more importantly added grip while holding.
With the new cover it's easy to pull the iPad out and see it's beautiful bare design, but I find it very slippery to use without a back cover and folding the keyboard behind means feeling the keys in the back which doesn't feel good.
I may replace it with Logitech's Slim Combo when they come out with one for the new size.