Neither is wrong, it just depends on your requirement. I wouldn't use DATEDIF at all for the days difference between two dates, use either
=C2-C1+1
or just
=C2-C1
Let's assume that C1 is today and C2 is tomorrow. If you want the difference to be 2, perhaps because your requirement is from the start of the first day to the end of the latter, then use the first formula
If you think the difference is 1, perhaps measuring from a specific time on one day to the same time on the other, then use tghe second formula