Hi caejam;
Melted chocolate and cocoa will act differently in your baking; while cocoa powder is just cocoa solids, chocolate is a combination of cocoa solids, cocoa butter, and sugar (plus some emulsifiers and preservatives, usually).
It's not too difficult to substitute one for the other; cocoa to chocolate= take out some fat and sugar from the recipe, and chocolate to cocoa= add some more fat and sugar to the recipe. The texture will never be exactly the same though, unless you're using cocoa butter to make up the extra fat - dairy, margarine, oil, coconut oil, and cocoa butter (all your baking fats) have different physical properties, and play slightly different roles in your baking.
Unless you're lucky enough to have a local chocolaterie that makes fresh stuff nearby, most of the baking chocolate you'll get will taste pretty similar; use the good quality stuff though, because you will notice the difference! Cocoa has a bit more variety, and Dutch process (smooth and neutral pH) vs natural cocoa (sharper, more acidic) will affect the pH of your mix (if you pay attention to that kind of stuff in your recipe formulation). Be sure you're using the unsweetened cocoa for baking, and not the chocolate milk mix. =D
Taste differences mostly come down to subtle personal preference (in my experience), what you're making, and the method.
Experiment with both, and see which you like better! Have fun =P