FSF say that copyleft is better (in most circumstances) for ensuring Free Software remains Free and also helps encourage the creation of more Free Software, but almost all Open Source is also Free Software by their definition.
"In the GNU project, we use copyleft to protect these freedoms legally for everyone. But noncopylefted free software also exists. We believe there are important reasons why it is better to use copyleft, but if your program is noncopylefted free software, it is still basically ethical. (See Categories of Free Software for a description of how “free software,” “copylefted software” and other categories of software relate to each other.)"
from http://www.gnu.org/philosophy/free-sw.html
There's a diagram here:
http://www.gnu.org/philosophy/categories.html
and a bottom line quote "the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free."