What counts the most for admission to a MSc : Erasmus programme or summer school / research programme? I'm doing my bachelor degree in Physics in Italy and I'm strongly considering to continue my studies abroad (in Europe) after the degree.

I don't know whether I should spend a period studying in a university abroad (precisely participating in what is called an "Erasmus programme") or apply for a summer school / summer research programme abroad during the years of my bachelor. 

Which of these two is more likely to be a parameter of valuation taken in consideration for a future admission to a master of science in a university abroad?

The Erasmus programme is considered to be a very good experience but I don't know if it is evaluated in an admission process, in the end it is "simply" a period of study, but in a different country. I mean, usually it doesn't add something to the formation of the student in terms of knowledge in the subject, which is something that a summer school or a research programme could do.

Does anyone have suggestions?

Thanks a lot for your help
Erasmus program is evaluated in sense that you are capable of living and working in new surrounding and that you can integrate easily and adapt faster to school system of given country ( host university). This mean that your advantage in overcoming so called "cultural shock" and that you will not spent you MSc with loosing your time and nerves in adapting.
but EP  is not crucial and never been, many students that I know, enrolled in MSc without Erasmus experience, 
If you want to pursue research oriented program ( which I highly doubt it exist in EU as a part of international MSc program, excluding UK) then Summer research program is desirable. I need to warn you, that quality of that research program and advisor on host institution needs to be appropriate.

As someone who been on both, erasmus and sumer research, I benefited more from summer research program, but I can thank to my advisor on that institution, also there are many examples where students found out that erasmus were more valuable experience than SRP, they felt like lab technicians during SRP. This you should avoid. Contact and inform well before going to that institution. I guess it is individual experience and each department or university that are in programs are not valued the same.