Online vs. store?

I recently made a list of a bunch of books I really wanted to start reading. Most are pretty old and can be found on Project Gutenberg for free. Most are foreign and from what I can see there are different translators all around. Are the online books just as good as, say, a Penguin Classics or Barnes and Noble Classic one? Books in particular include Karl Marx's Das Kapital and Nietzche's The Birth of Tragedy and Thus Spoke Zarathustra. Does a certain translation/edition "ruin" the experience?

Most free online texts originated from actual print books. Any translation you read probably came from a translation in a book.