Both VI and V are available on steam currently, and while the game series gets lots of praise (and money), it also gains lots of criticism for reusing the same games over and over, or being optimized. So lets look at the more basic reasons as to buy one over the other, while ignoring some of the more nuanced game mechanic differences that would only apply to individuals who play 1000's of hours compared to my measly 50-100 or so in each.
Graphics, art style, UI, etc:
While the graphics are different in that Civ V is trying to be realistic-ish, and Civ VI is cartoony, there are many differences in the UI and interface that make the games different. To put it simply, Civ VI does every UI element better than Civ V does them. It is an improvement in every way, from showing the limits of movement for troops by default, to having better navigability in the UI. One main problem with V is that even the UI is buggy, and the top banner will sometimes disappear for players. This leads us to...
Bugs and glitches: Civ VI has, from what I have observed, almost no serious bugs or issues. There are many balance issues people disagree with, but blatant problems with the games code aren't very common. After playing Civ V in singleplayer, besides UI bugginess as mentioned above, you might be inclined to say the same, however once you make the leap into multiplayer, the game essentially breaks down. Buttons stop working, turns can't be ended, players get kicked from the game, the list goes on and on. Having more than 5ish people playing also makes the game slow to a crawl, for some unknown reason, if any of the people playing are on lower end pc's. To put it simply, the game is fun, and has cool mechanics, and when working is a blast, but after a while the bugs start to become more than just annoyances. Players on macs when the rest are on pc's often cannot even join the game.
One major drawback of Civ VI is macs cannot play with PC's running windows. This may seem like a deciding factor to buy civ V, however note that cross platform play hardly works anyways in Civ V, so you might as well get the game that works otherwise.
Content and gameplay: for the more casual player, the only real differences are the new policy tree in CIv VI, the new worker mechanics, road mechanics, and the districts. The districts make the game that much more interesting and require a bit more planning if one wants to master the game. Civ V, however, has all the DLC's one could want, all bundled for a low price on sale. Civ VI has very few Civ's in comparison, and the first expansion is not quite released yet, almost 2ish? years after the game first came out.
Overall, Civilization VI is the much better game, and the multiplayer actually works consistently, which is the biggest takeaway. It is more expensive, but worth the buy over V.