eightthirtytwo

Of course, what really matters is the relevance the chosen language to what you plan to study, but it's also a good to consider the fact that German will open up your linguistic horizons way more than Italian. What I'm trying to say is that due to the relationship between French and Italian, if you needed the latter for research down the road, you could take a reading course and work your way through. German is so completely different. Taking German would open you up to Dutch, Scandinavian languages (to a limited extent) and some others. Overall, I think German would be more practical, but I study Northern Ren so I'm biased. I'm currently giving myself a crash course in Dutch which would be much harder without the German minor I did in undergrad (I also speak French fluently so).
tl;dr think of your art interests, the grad programs you might want to go to (UPenn requires German AND French, I believe), and what other languages you might need to work with

In general, if you are interested in Northern Renaissance, then German would be helpful. If Southern Renaissance, then Italian. I know a lot of phd programs require 3 language, mostly French, German, and another language that is appropriate for your area of study. For MA, it's mostly 2 languages - German and French. Sometimes schools waive one with Italian or Spanish, but not all. 
But this is just what I've heard, and I am sure it varies from programs to programs. 

One upside to German is the easy availability of money. The DAAD (Deutscher Akademischer Austauschdienst) has a very ready supply of funding for students learning and/or doing research in Germany. Obviously there are comparable opportunities in Italy, but with nowhere near the same level of funding, and with a lot more competition.