I would say get the United card because the United Mileage Plus miles appear to have much more use whenever I tried to book an award ticket in the US, to Asia, or to South America. On the other hand, trying to use my connectmiles to book an US award ticket was twice as many miles required than what I could get on United miles, or no results were shown.

Case in point:

I was trying to book a trip from Asia to South America using miles. During the time I wanted, United wanted 55,000 miles one way plus taxes while no availability showed when I tried with my Copa Connect miles.

When I tried to book a one way ticket from US to Canada, United wanted 13,800 miles one way whereas Copa had it for 25,000.

On the other hand, having Silver with Copa is good in that I was upgraded more often than anticipated as Silver when flying on Copa's routes.