According to Xperia Blog, both the LTE (SGP771) and Wi-Fi-only (SGP712) versions of the Xperia Z4 Tablet are being updated to Marshmallow. As for the Xperia Z3+ smartphone, at the moment, the update is live only for the single SIM variant (E6553), though the dual SIM model (E6533) will likely get it soon, too.
Sony is officially selling the Xperia Z3+ and Xperia Z4 Tablet in the US, though it's offering them only unlocked, via retailers like Amazon - so you can't get them from any carrier. The Xperia Z3+ can be purchased for as low as around $400, while an Xperia Z4 Tablet will cost you at least $600. Would you buy any of the two devices, now that you know they're being updated to Android Marshmallow?