Take a look at this for comparison of the different versions: http://msdn.microsoft.com/en-us/library/cc645993(v=SQL.110).aspx
How active will the system be and how large is the database going to be? I think the 1GB RAM limit should not be an issue.
Also, having 70 concurrent users shouldn't be an issue either. Take a look at this for additional info: http://erikej.blogspot.com/2011/01/comparison-of-sql-server-compact-4-and.html