A 'J'-pole is probably one of the most common 2 meter ham antennas around, after a 1/4 wave groundplane antenna. A 'J'-pole is a 1/2 wave antenna. There are differences between the radiation patterns of a 1/4 wave and 1/2 wave vertical, where a signal is sent to or where it comes from. Which would be 'best' depends entirely on the circumstances of use. It's a fairly safe bet that a 1/2 wave antenna such as the 'J'-pole will have a slightly larger radiation pattern, will tend to 'pick up' more than a 1/4 wave groundplane antenna, but it's not a 'sure thing' at all. What would be the difference in your particular circumstances? I would have to guess that there would be some improvement in performance. The thing there would be if it was worth the time/effort/cost of switching antennas, and I can't answer that.
There's no particular benefit in copper as an antenna, other than it's easier to work with than other metals. There would be no practical electrical differences between using copper or steel.
I've used both types of antennas, they both perform well if tuned well. If it's a choice between a 1/4 wave and a 1/2 wave, I'd pick the 1/2 wave. If given another choice, I'd rather use a 5/8 wave vertical. That's because of my particular circumstances. Your milage will vary... different circumstances.
- 'Doc