This topic is currently being hammered already in multiple threads. Just look at virtually any thread under iPad and iOS 11. As should be expected, the 12.9 will use the features “better” because of the bigger screen. If space is your concern...12.9. If portability is your concern...9.7. If you want a rather “sweet spot” between the two...10.5. Gen 1 and 2 of the 12.9 as well as the 10.5 handle the multi-tasking similarly minus a couple minute details.