What are you studying?
How old are you?
What languages do you speak? How well do you speak them?
What is your budget?
What are your interests?
What are your boundaries? Does anything, in all honesty, freak you out?
They are both wonderful, amazing cities. Almost everyone in Berlin speaks English to some extent. It is an exciting, vibrant city that is easy to navigate. COL in Berlin is still quite low for a major European metropolis (hehe). One thing to keep in mind: until relatively recently, it was two cities, so it can take a little longer to get from point A to point B in Berlin. Things can be far from each other. It is incredibly cool, and funky, fun, and artistic, with old spaces being used in new and innovative ways. There is also an acceptance, no, an embracing, in Berlin of all kinds of difference. It's quite inviting that way. You never feel like a foreigner there -- it's like NYC in this sense. The accent, to my (Spanish, English, French trained ear) is the easiest German dialect to understand and pick up.
Venice is beautiful and smaller and easy enough to navigate. They all speak Italian, some speak Venetian, and many speak English, since it's a tourist town. The artwork is impressive, and the vibe is something else. It is more expensive, and I am not sure what it would be like there as a student. As an adult, it is so incredibly, awesomely beautiful, with so many beautiful churches, museums, restaurants . . . Italy in general is easier for me, because I speak two Romance languages already. Hmmm, decisions, decisions.