Semi-sweet or dark chocolate for baking:

You might find this analysis of dark chocolate of interest: http://www.cooksillustrated.com/tastetests/overview.asp?docid=11873&Extcode=N1NSPFA00

When I wrote "baking" above, I was referring to instances where chocolate is a part of the chemistry rather than chips in cookies. Cocoa solids and sugar have opposite properties when mixed with liquid ingredients; fat content can influence the outcome as well. There's a parallel with cocoa in baking -- it's important to distinguish between "dutched" and so-called "natural" powders.

Agree for the most part with chefono, especially regarding the no substituting of chips for bars when melting chocolate; you can, however, flip the other way.

Otherwise, I think a lot of this is about taste. If you're making something like a flourless chocolate cake or a mousse--something which really showcases chocolate--you'll want to use a richer chocolate that is of the best quality you can afford. But in many things like frostings and cookies, you can really get away with using what you want, regardless of what the recipe dictates. I like mixing milk and semisweet in chocolate chip cookies, for instance.

One manufacturer's bittersweet is another's semi. About all those terms tell you is that you're not dealing with milk chocolate. Even bars with % cacao on them don't tell the whole story as the figure represents a combination of both cocoa solids and cocoa butter. How much of each is anyone's guess. And nothing will tell you if the cocoa was processed with alkali or not (or in the case of Hershey's Special Dark, a combination of both which is part of what makes it stand out from the crowd).

In baking these variables can matter a great deal which is why a properly written recipe will specify which chocolate to use. As a general rule, you should be safe between 60 and 70 percent (do the math if you combine two types). Use caution with bars labeled bitter or semi -- they can vary from 35% all the way up to 98% (although most will be on the lower end of the scale due to economics).

Whatever you do, stick with chips if the recipe calls for chips and bar chocolate if you're melting it. The two may look and taste the same but chips are engineered to *not* melt and fall apart in cookies and such.

for me it depends upon the recipe. I prefer the flavor and intensity of bittersweet chocolate, so sometimes I use that even when semisweet is called for by the author. And for cakes I usually go with unsweetened.

Can you tell us more about the recipe? Is this a brownie, in which you need to melt down the chocolate? A cookie?
For cookies, I use all different grades of chocolate, depending on the goal--but most recipes will call for semi-sweet. I like a dark chocolate chip cookie and I use Guitard, I think it's 62%.
Brownies, on the other hand, will call for baking chocolate, which is bittersweet, likely about 58% or above.
If it were me, I'd save the highest grade chocolate bars for eating and melting down to make ganache.