This is one of those things where if you have to ask then clearly your level/frequency/intensity of saw use is such that either will work fine for your needs.
There are crappy aftermarket bars and chains, but if you stick with the pro-level goods from the big names, they will all get the job done. Some bars are more flexible than others, with harder or softer rails, and weigh more or less than another brand's bar of the same length. These aren't good/bad sort of comparisions, they're just different. One guy might want a heavy, hard Cannon bar, and another prefers a softer and lighter Oregon bar for his needs. Same with chains - a guy who hand-files all his chains might prefer the softer cutters from Oregon because they're easier to file, or he might prefer the harder ones from Stihl because they hold an edge a bit longer. It's about having choices in the marketplace, that's all.
MCW's answer also adds some really good points!!