Do you already have an Android or iOS device? Stay with the system you have.
Do you have both? You should be able to figure out why you would prefer one or the other in a different screen size.
No tablets, no smart phones at all? Your choices go beyond iPad and Nexus, because if you use Windows computers you might want to consider a Windows tablet. Those are back, and Microsoft is selling one themselves thos time, on the premise that previous market failures were because no vendor did it right.
I chose iOS because it is a closed system. Many people hate it for that reason, but thirty years computing, software development experience in closed (IBM, DEC, Apple) and open (UNIX, DOS) systems taught me to choose open systems when I want to play around, closed systems when reliabilty and stability matter.
What matters to you? Freedom or trouble free?