Depends on what is important to you. If you are a gamer and really only care about gaming on the computer then a better GPU is probably the way to go. If you use your computer for work as well as gaming and do equal parts of each, a second monitor will do wonders for your productivity.
alright, i think im going to go with the better GPU for now, and upgrade to a second monitor as soon as possible afterward. i will mostly be using this for gaming but i do also use it for work purposes.