He's the most comparable talent as far as age (less than 1 year apart in age). It's a reasonable conversation to have.
I think knowing what I know right now, I'll take Tatum for the long term. His defense over Doncic's is cumulatively better than Doncic's offense is better than Tatum's. Doncic doesn't have the tools to ever really be that good on defense. Offense is great for the regular season, but you need defense over 7 games to win in the playoffs. Offense in the playoffs is heavily scheme and matchups based. They both unlock a lot schematically, Doncic obviously more with his playmaking. But Doncic will get abused by good teams on D, whereas Tatum is all defense level.
Tatum's playmaking has grown by leaps this season. Doncic's shotmaking should improve to get close to Tatum's (because right now Tatum is better in that area), and I don't think Tatum's playmaking and vision will ever get to Doncic's level because it's mostly instinctual. So I definitely see the other side. But Tatum's shotmaking let's him play off the ball and to help the team when someone else has the ball or he's struggling/has a bad matchup. He fits in with more teams, whereas Doncic is going to need the ball (and rightly should be given the ball in most circumstances) to really do anything, until his shotmaking improves.
Doncic fought the Clippers hard (and while injured!). But maybe the Clippers weren't actually as good as we thought because of coaching? Whereas Tatum was the clear best player in the Raptors/Celtics series with a coaching disparity of Nick Nurse vs. Clippers' Doc Rivers. Carlisle vs. Stevens is pretty comparable as far as their own teams go.
To summarize, I trust Tatum's defense to be the difference at the highest, highest level of play, while I worry Doncic will always be a liability or at best neutral there. Meanwhile, I do think Doncic will probably lead his team to more regular season wins on a consistent basis and will put up better stats and lead the league in a lot of metrics for a long time to come. I think Doncic will raise a team's floor more and Tatum will raise a team's ceiling if they're already good, and I find that to be more important. But I do totally get if someone disagrees with my assessment. Doncic is unbelieveable and may be able to work on his body and get better on D, but he's been a pro for like 5 years now and hasn't really, so I'm skeptical and he'll just never be as long as Tatum. Basically, I think Doncic will be closer to Harden than LeBron and Tatum is closer to a modern Kobe. Tatum will need his Shaq/Pau/whomever, but is there really a 2nd player that puts Doncic over the hump? Maybe a Adebayo/Draymond type, but those guys are RARE.