From what i can see on the iPads i have used, not all apps do allow split screen functionality. At least on the 10 inch models, no idea if it is different on the bigger one. Importantly, quite some basic EFB apps do not allow to use split screen, like the FlySmart suite from Airbus, our document library app. LIDO on the other hand works both in slide over and in split screen. The slide over thing works with apps that do not allow split screen, for example i can check in a slide over LIDO window the distance on a runway chart for an intersection performance calculation. But to be honest, in most if not all cases the use case is one app at a time. The size of information displayed can be, thank god, usually very easy resized to fit ones ability to read
Would i take the larger one? Of course, but on the other hand i do understand that there needs to be a business case, especially if we talk about thousands of devices for a normal pilot pool.