What counts the most for admission to a MSc : Erasmus programme or summer school / research programme?
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.