From what I can gather the only difference is the Z380C has an Intel processor and the Z380KL a Qualcomm processor. Which is better I've no idea.
But you can see the Z380KL got Marshmallow (6.0) in 09/2016:
https://www.asus.com/Tablets/ASUS_ZenPad_80_Z380KL/HelpDesk_Download/
As far as I can tell the cheaper Z380C variant is still suck on Lollipop
. The Z380C is now really cheap but Lolllipop is old and Marshmallow brought with it noticeable battery improvements.
Thanks
But you can see the Z380KL got Marshmallow (6.0) in 09/2016:
https://www.asus.com/Tablets/ASUS_ZenPad_80_Z380KL/HelpDesk_Download/
As far as I can tell the cheaper Z380C variant is still suck on Lollipop

Thanks