How reliable are QS Top Universities rankings?

I was told that universities in USA are generally better that the ones in Italy.

This statement, in its given generality, is false. The US, like most places, has very good schools, ok schools, and a reasonable number of pretty bad schools. If you explicitly are looking for very cheap schools, I would not assume that the remaining selection is necessarily any good, just because it is a "school in the states".

POLIMI is a pretty good school in CS. To be honest, if I had the choice of going either to POLIMI or to a low-ranked US school, I would almost certainly choose POLIMI.

The linked QS world university rankings are a well respected international ranking. That said, using a university's rank is a really terrible way to chose a grad school. Some of the factors which drive the rankings are important for choosing a grad school, but don't just rely on the rankings.

If you are planning to pursue PhD studies in computer science, you should in general not have to pay tuition yourself—this is covered by either your advisor or the department as a whole (e.g., if you're a teaching assistant).

So, basically, it comes down to a question of whether or not you're able to gain admission to a school that you think is better than the one you are attending in Italy. Equally importantly, if you're looking to start this fall (2014), you will almost certainly not be able to do it in the United States, as the admissions "window" for fall of 2014 has already closed.