Choice is the best and for brisket it pays to get it. The only brisket I have done sous vide is corned beef. A little different. Look for the one with the most marbling.
If you are buying a whole brisket I would recommend that you first trim the fat from the bottom and all but a quarter inch from the fat cap on top. A brisket is actually 2 pieces of meat. The bigger bottom piece is the flat and the smaller fattier piece is the point. They are separated by a layer of fat. Personally I would treat them as 2 cuts of meat and separate them especially if you are going to make steaks. The flat is often injected with beef broth to add flavor and moisture.
I would look for a recipe for burnt ends for the point. These are the prized part of the brisket. There also should be recipes for the flat. I have only smoked brisket so I will let you look for a sous vide recipe that looks good.
Here is a picture of a brisket
Since they are cheap you may also want to consider making your own corned beef and pastrami (smoked corned beef). Corned beef sous vide is the best I have ever had.