Check out http://www.archi-students.org
if you haven't done so already - you should find some helpful information there, as there are normally students in different European countries discussing Erasmus opportunities in the various forums!
Although, I have no personal experience of the Erasmus programme, one of my college friends & I did discuss the programme with the Head of our school, as a friend of ours studying in ETSAB wants to come to our school. The Head basically told us that in order for a student to go on Erasmus to a particular school of Architecture, a bi-lateral agreement must be in place between the two schools. So perhaps you could check with your Erasmus co-ordinator to find out what schools of Architecture your school is already in agreement with, as it might help you decide & if you don't think they are suitable for you, then you could look into getting a bi-lateral agreement put in place (the Head advised us that the Erasmus co-ordinator in one school has to send a written request to the other school for a bi-lateral agreement to be put in place)
We also discussed the advantages of Erasmus with him & he advised us that it is an excellent opportunity for students. He recommends that students go on Erasmus in 4th year and it is normally up to the students own school to decide whether or not to send a student on Erasmus, depending on how a student is progressing in their studies. He advised that it is normally easier for a student to go on Erasmus to a school where they can speak the local language as studies are usually conducted in the local language and therefore, there is a lesser risk of the student falling behind!
You are probably aware of most of this already and I apologise if that is the case - if not, I hope some of it has been helpful!