If you only just started Civ 5 i'd say stick with that for now.
Is Civ 6 worth a 25$ price tag ? Yes it is.
But even more worth it (imo) would be the 17$ for which you currently can get both of Civ 5's major expansions "Brave new world" and "Gods and Kings (currently 7,49€ each)...if you don't already have them.
There is always another sale, Civ 6 isn't going anywhere and you can always get it later, and possibly even cheaper.
Civ 5 is a very good game in the series, i dont think you'll regret giving it your full attention.
Personally i also think it's the better game of the two, but that's highly subjective of course.