I'd say "su -". Both should work though.
Basically, when you are installing something and you need to run ie "make install" as root,
one of the things it does is copy the compiled application (compiled and linked via "make" typically) to
standard paths like /bin, /usr/bin, etc.
However, those directories are readable/searchable by everyone (so anyone can use the utils), but they're only writable by root (thus making root privilege necessary for installing stuff).
Changing these permissions to make the directories writable is a serious security hazard.
However, your environment settings probably don't play a role when "make install" is copying programs into
/bin, etc (cp only needs to be run under root user to have the write access).