Choosing between iPad Pro sizes
There’s a definite trade-off between size and portability when choosing between the two sizes of iPad Pro. The 10.5" version is lighter and more portable—it’s essentially the same as the familiar 9.7" model. The 12.9" version is heavier and has a larger screen, and, unfortunately, has larger bezels as well (it is the same size as the prior 12.9" model).
The smaller iPad Pro is nicer to hold in your hands for long periods of time. Even more importantly, if you read books on it in bed, where the screen is only about 12" away from your eyes, as I do, it is easier to read on. Because you can focus on the whole screen at once, you don’t need to move your eyes as much. It is small, but it is perfectly usable for productivity as well as videos and reading.
The larger iPad Pro is, in my opinion, an even better size for getting work done. The 12.9" screen, when paired with a hardware keyboard, feels like a computer screen for productivity apps. It’s barely smaller than my old 13.3" MacBook. There is plenty of room for multitasking and slide-over. It also feels even better for handwriting and drawing, because the usable screen area is basically the same size as a standard piece of paper (i.e., A4 or Letter size), which feels natural.
Does the additional size and weight make it less useful as a tablet? In short: yes, but only a little.
The 12.9" iPad Pro (2017 model) weighs about as much as the iPad 4th generation (about 1.5 lbs), so weight is not really a problem for handheld use, at least when held without a case. Apple’s Smart Cover comes off easily, so you could just take it off to read with for long periods of time, and put it back on when you want to protect the screen. Also, the weight is very well balanced, so you can hold it in one hand comfortably despite its size.
As for reading in bed or in an easy chair, the larger model is slightly worse. It’s more natural to read such a large screen at an arm’s length away, rather than perched on your stomach or thereabouts. The large screen has the same DPI as the smaller iPad Pro, so crispness at that distance is not a problem. It’s just too big to focus on the whole thing evenly, unless it’s about as far from your eyes as your laptop screen normally would be. (This is, obviously, a very small problem in the grand scheme of things.)
It's easy to have no trade-offs when you can just have both
One reason behind my purchase of a 12.9" iPad Pro was that I realized that I didn’t necessarily have to give up my old 9.7" iPad Air after doing so. Sure, it might be the smart thing to get back $100 or so by selling it, or it might be nice to give it to someone else in my family, but it is still worth a lot more to me than it is to somebody else. (I admit, it does help that my kids are too young to care about electronic devices.) This decision made it much easier to go for the larger-sized tablet.
So, if you already have a smaller-than-12.9" iPad, like I do? Honestly, if you can get away with it, keep the old iPad around for reading and use the new, larger one for desktop use.