The 12.9 felt like apple finally found the right size for the iPad to be. A 15" might feel even yet more righter. From a performance standpoint it's a beast, and feels like the first iPad suitably future-proofed to not slow to a crawl in a single update cycle.
However, it is quite heavy, and large enough to not be very portable. ...which is fine, if you consider it more idealized as a creative platform than a mobile device. ...but then it has a mobile devices OS, and iOS's selection of apps, whose functionality is so limited by being designed as mobile apps, that it made all our architects, designers, & engineers wish Apple would combine this great hardware with their great professional OS, into a capable, powerful truly professional creative platform.
Even yet more righter.