"Da Samurai" is a blowhard samurai wannabe braggart who is all talk, all presence, and no action, showing up and making a scene of himself hyping up to be the best samurai ever. He was later humbled by Samurai Jack, showing him the ways of being a true samurai warrior.
David Alan Grier is absolutely amazing as Da Samurai, making the best of his one episode by playing up his eccentric showoff personality, bragging about himself to everyone and hitting on ladies, spitting out lines in such a manner that leaves an IMPACT.
Keegan Michael Key plays "Da Samurai" in Season 5, far into the future after he was humbled by Samurai Jack, now running the same bar that he first met Jack. While I don't have doubt that Grier would've played an older version of Da Samurai as well as Keegan, Keegan doesn't do a bad job at capturing the hip, jive and quick personality that Grier was given, while adding his Key & Peele flavor.
In the end, both play him at different times and settings well, but I think Grier gives off more flavor and a more playful attitude which is just.. so very memorable.