These are the best quickdraw slings for sport climbing. They are wider and heavier than the thin dyneema slings. This makes them easier to grab and hold and less likely to twist when pulling them off your harness, placing, etc. If you are shopping for this type of sling you probably already know this though and must decide between Petzl Express and BD Vari-width.
The Petzl Express stands out on several minor points. The rubber stay on the rope end of the Petzl Express acts as a carabiner stabilizer and protective sheath for the sling. This can be important if your quickdraws stay on the route all day, or where I climb, for months at a time, as they swing around and contact the rock the point of wear is normally the area of the sling around the rope end carabiner. This area is protected by the rubber stay on the Petzl sling and will enhance durability. Not so on the BD, the rubber stay on BD slings is inside the sling. Yes, the rubber stay will wear more quickly on the Petzl, but the rubber is not load bearing.
On the 17cm length (18 for BD) the Petzl bolt end opening seems tighter than the BD so may better restrict the sling from traveling up or down the carabiner to avoid potential cross loading. The tighter opening may also reduce the Petzl's tendency to twist. The Petzl bolt end opening is not snug though, and is still plenty loose enough to allow the sling to travel back to the seat of the carabiner in the event of a fall if it has managed to move up the carabiner while climbing. Note both brands include an extra stitch to tighten the bolt end opening on the 25cm length.
Finally, the weight, the Petzl is lighter in all three lengths. According to their respective websites, the Petzl 11cm is 10g, 17cm is 15g, 25cm is 20g (REI lists 14g, 17g, and 23g) and the BD 12cm is 16g, 18cm is 24g, and 25cm is 34g. This weight difference is significant and probably due to the width; the BD is 27mm wide, the Petzl is 19mm. If you have huge hands, the 27mm BD may be a better handle, for me the 19mm Petzl is a handful.
I use the Petzl Express as my sportclimbing quickdraw sling with BD positron straight gates on the bolt end and BD Hotwire on the rope end. The carabiners on either end are debatable and negotiable, the slings are not. 5 stars.