Which seat is better for the DR650?
"Better" is a relative term. All of the aftermarket seats are a huge improvement over the stock seat which provides little support.
The Sargent is a few pounds lighter than the Corbin, but the Corbin, while being a bit heavier is built like a tank and will likely outlast the motorcycle. While the Sargent is definitely well made it doesn't give that bulletproof impression like the Corbin does.
The Sargent is a bit narrower in front which is nice when standing on the pegs. The Corbin feels wide between the knees when you are up on the pegs. Being narrower up front the Sargent doesn't give you as much support if you tend to sit more toward the front of the seat.