The Air Canada MLL at EWR is located in terminal A concourse A1. Not at all convenient to get to unless you are departing from those gates as each concourse has its own security checkpoint:
https://www.united.com/web/en-US/con.../maps/ewr.aspx
As for the UA clubs located in terminal C: the club near gate 120 is larger, has tarmac views and showers, but gets extremely crowded. The club near gate 80 is generally less crowded as it is located away from the majority of the concourse C gates.
As to your general question: both the UC and MLL are considered equally mediocre, but without being more specific about what is important to you in a lounge experience it is difficult to be more definitive.
SL