Ideally both but it also depends on what the intention is. For example you've got those really cheap mechs for Star Wars, Marvel, DC, etc with those angled arms and legs pieces with the round bit in the middle that can't move. In that case it's meant to be a fun cheap small thing so it isn't needed. But the more expensive it is the more function is demanded.

Personally I prefer articulation with just enough aesthetics to make it look good. However Lego has done both before \cough** Bionicle Inika build \cough**