In practical terms, there’s not a lot of difference. The quality of the chocolate you use is more important. You might find as much flavor variation in two different brands of bittersweet chocolate as you would between a good-quality semisweet and a lesser bittersweet. By definition, bittersweet chocolate has at least 35% chocolate liquor. Semisweet chocolate can have up to 35%, so they can be virtually identical.
If you’re using gobs and gobs of bittersweet chocolate in a recipe that calls for semisweet and it turns out less sweet than you like, add a little extra the sugar next time you make the dish. (Many connoisseurs (or food snobs) would prefer the bittersweet version anyway.)