Doesn't it depend on what you are making? One of the things I remember most about cooking school was the chocolate tasting (11 dark chocolates). We had to assess each for sweetness, fruitiness, etc. Some chocolates taste more fruity, some have more acidity, some have more roasted bean flavor, it varies with the maker. I'm pretty lucky to be able to use valrhona in my work. Most of the time we use Manjari (64%) for our desserts because it has great fruitiness and edgy acidity. But for our cinnamon chocolate mousse, we use Caraibe (66%) because it's fruitness is subdued, meaning we can taste the chocolate more and it allows the cinnamon to compliment rather than compete with the chocolate. We had some Callebaut left over from the previous pastry chef, so we used it in our chocolate ice cream. When I was given a piece to taste, I have to say that the Callebaut seemed to be boasting it's roasted character. It had controlled acidity and I couldn't taste much fruitiness. It made pretty good ice cream.
I also agree that it's the cocoa butter that affects goopiness when melted. But I also think that it has much to do with the conching process. Callebaut, scarffenberger are both superconched. You can tell when you put a piece of it under your tongue and it takes forever to melt and mingle with your saliva. But Callebaut is much more goopy when melted than Valrhona. At another restaurant I worked at, we used michel Cluizel for something (I can't remember), but that melted super thin. To thin, we usually use either cocoa butter or butter depending again on what we use it for. Cocoa butter for chocolate candy fillings and butter for pastries.
Shroomgirl, you are a serious chocolate connosieur! I can barely eat Valrhona 70% (I think it's Guanaya) straight or mousses made with it. And it was tough to work with. We had to add butter to liquify that one a bit.