They are not completely similar players I think. Kante is more like a ball winning midfielder. Conte played him alongside Fabregas in a 3-4-3 variants last season and this season. He shines in a two man midfield. He's like a CM with DM instincts. He wins the ball while opponent is trying to settle in Chelsea's halfway, then quickly pass it over to Fabregas, Hazard or another creative player. In a possession game, Kante freely roams around central midfield area, ensuring that return balls from Chelsea attacks are collected by him and distributed to wide players like Alonso or Moses, or Fabregas, Hazard, Pedro etc. Kante is very effective in Chelsea system, also in general counter attacking tactics.
Let's look at Casemiro. Casemiro shines in a 3, recently 4 man midfield. He fits Real Madrid's system like a glove. He is good in both possession oriented system and counter attacking systems. Also, unlike Kante, he's more of a DM with CM instincts. When Madrid dominates opponent's midfield and controlling possession, Casemiro roams around "entire" midfield, that's encompassing DM, CM and CAM areas, both narrow and wide. One second he's next to Ramos, next he's around the box collecting the return balls, next second he's inside the box waiting for a low cross from Marcelo lol. His workload is actually heavier than Kante in my opinion. But the thing is, I used to think he's just an average DM. But then I realized that he's actually a silent genius in our team. He's the player of "total football" philosophy. That means he's not a specialist that's only tasked with collecting return balls or cutting down opposition counter attacks like an ordinary DM. Day by day, he's progressing more and more into a Pirlo-esque Regista role. He starts as a DM, but freely moves in the pitch, makes a threatening runs in some phases of the play. That's the total football philosophy, that a player can be involved in multiple phases of the game, carrying multiple roles. That's why you see him surprisingly scoring against Juve, United etc. with "different" types of goals.
In possession oriented play, Casemiro switches roles with Modric and Kroos. They take turns in covering each other and in defending, supporting Madrid attacks. Casemiro freely roams around, while having an eye on possible opponent counter attacks, he also eyes for return balls and goalscoring opportunities. In counter attacking play, Casemiro actually has a very good vision. Look at his assist to Ronaldo in Bayern match last season, in Bernabeu. He wins the ball around the halfway line, quickly crosses it to Ronaldo, who is waiting inside the box and boom, goal.
I think in this comparison, Casemiro actually has more quality and can carry more roles. I'm sure Pep would've killed to have a player like Casemiro in his team. I'm saying this now, watch out for Brazil in the world cup just because of Casemiro. Dude will score, assist, intercept, tackle, pass, cross etc. will do everything.
And lastly I can't believe I'm writing these words about Casemiro. Why? Because he wasn't like that for the most part in the past. He was just an ordinary DM with good potential. But under Zidane he's going through an insane transformation. He's not an end product yet, mind you. He's going to be scary good in 1-2 seasons, going by this rate.
So, all in all, I would prefer Kante if I had limited options in my squad tactically. If I have a squad that can play football in multiple ways, be it possession oriented, high pressing, or counter attacking, I would pick Casemiro all day, everyday.