The Chuck is the more traditional pot-roast,  a slightly tougher cut of meat that has a nice amount of marbling - anything from the chick and shoulder area have more flavor. 
 
The Sirloin roast is our favorite roast.  It is fairly tender and not so fatty as the chuck and shoulder roasts.  I love the flavor it has, DH likes the texture better.  It will hold together for a nice slice of roast beef, leftovers are great as French-dip sandwiches.
 
I'm not really allowed to make Pot-Roast or as my husband calls it "Stingy-Beef".  Definately not one of his preferred meals.  In fact, I was told that the Oven stew from last week was not as good as usual because I used the "stringy beef" instead of the beef roast I usually use.  So now the poor Chuck Roast is for pulled-beef type recipes only - B-B-Q Beef sandwiches, Italian Beef or Mexican Beef Tacos.
 
When the local grocery store had a sale on Sirloin, I picked up a whole sirloin in the cryowrap bag for $4.99 a pound and cut my own roasts out of it.  I also had them grind a whole one up for burger - the burger comes out to be 90% lean that way and I know it is fresh.