How to install MacPorts on user-only account?
Neither.

Macports is installed as root and uses its own account macports for some things. It does not use your account for anything.

To do MacPorts actions e.g. install, selfupdate you need to run it as root. Which in this case is run via sudo. So to do these actions you do need an admin account.

To use MacPorts executables or libraries you just need to add MacPorts to your user environment, which is the same for any user on your machine. e.g. add /opt/local/bin to $PATH to put the executables on your path, as described in the MacPorts manual.

The one edge case is the install from the disk image which will run a script on your current user to add a line to your .bash_profile to set up your path.
You can also manually add the following line to your Admin's .bash_profile:

export PATH=/opt/local/bin:/opt/local/sbin:$PATH