I think Tatum and Brown are about good defenders but for different reasons. Tatum is great at being beat, and coming behind for the chase down block, and using his height + length as an excellent team help defender. Whereas, Brown is probably better at guarding stronger forwards, but has defensive versatility from 1-4, and is a better man to man defender.
Lol at the bolded part! 
Btw, Tatum isn't all that long. He's 6'8 with a 6'11 wingspan. That's about average length in my book.
I believe Tatum is an adequate defender when it comes to team D. He's a below average defender when it comes to on-ball D though.
All things considered, I don't think he's a 2-way player like Brown is.
That was more in jest 
But I disagree with adequate defender part.
Tatum's been 2nd in RAPM, 5th in RPM, and 12th in PIPM.
I've noticed that Tatum generally sort of lays back, and allows smaller point guards or shooting guards/forwards to blow by him, but he'll somehow have the ability to contest or block the shot from behind.
“I can’t believe it’s not talked about more, how good he is defensively,” said Stevens. "I think, for whatever reason, that gets lost in the shuffle. How much effort he’s been playing with all year has been like — he’s really become a great defender. His length. He chases balls, he challenges shots. He gets his hands on balls or keeps them in their mind that he’s behind them with that length. And then he’s a great rebounder from the wing.
"So he’s a really good defender. He’s a big reason why our defense is where it is as a team and we need him to keep continuing to be at that level.”
The Celtics, pegged to plummet in the post-Horford/Baynes era, now own the fifth-best defense in the NBA while allowing 103.6 points per 100 possession. Quantifying an individual player’s defensive impact can be tricky and much of the defensive metrics publicly available tends to be noisy.
But take from this what you will: The NBA’s defensive tracking data, as measured by the cameras mounted in all 29 arenas, has Tatum holding his defensive assignments to 36.9 percent shooting this season, or a whopping 8 percent below those players’ average for the season. As a means of comparison, Kawhi Leonard — widely regarded as maybe the best perimeter defender in the NBA — is holding opponents to 38.1 percent shooting, or 8.5 percent below their season average.
Data-tracking website Cleaning The Glass offers a percentile rank of a player in various defensive categories relative to their position. Tatum ranks in the 89th percentile while fouling on only 2.3 percent of his team’s plays, and ranks in the 85th percentile while registering a steal percentage of 1.8.
Then there’s Tatum’s rebounding: He’s grabbing a career-best 7.1 rebounds per game and his rebound percentage is up to 9.7 percent overall (up 1.6 percent since his rookie season). While there’s a lot more pressure on Boston wings to rebound given the personnel that left, Tatum has been a willing glass crasher who again uses his size to get to those balls.
"He's been a good defender since he got here and I think he's just improved at the rate you would think that a guy in his third season should improve,” said Stevens, reeling in his praise a tiny bit. "His ability to guard guards off screens, especially off-ball screens and impact them with his length has been really important for our team defense this year. We can end up putting him on some of those guys so that Kemba is not in every single one of them.”
The 39-point nights are nice. But Tatum’s ticket to stardom is being able to impact the game beyond the point total.
"I’ve said it before, want to be one of the best players in the league at both ends,” said Tatum. "Guys like [Paul George] and Kawhi are really dominant on both ends. Been trying a lot this year to match my intensity on offense with defense.”
And he’s doing it.
The team defence is absolutely elite, but the bolded parts need context: Tatum's role as a man defender is to take on the easiest defensive assignment among our trio of wings, which makes his stats as a man defender look really good since he's not a
poor man defender (I peg him as average to slightly above average).
As for how his defence stacks up against Jaylen, I think Brown is an elite man defender and a good team defender (his mental lapses prevent him from being great or elite, but he's still very disruptive when his motor is revving due to his combination of length, speed, explosiveness and strength), which puts him around Tatum's level if not higher imo. I'm aware of the fact that team defence is inherently more valuable than man defence, but the difference between their man defence is larger than the gap between their team defence, ntm that Tatum is less versatile than Brown on defence (Tatum really needs to play that free safety role to maximise his defensive value, while Brown can be a major contributor on defence in any type of role, whether it's as a point of attack defender or as a team defender).
