I built a J-Pole for MSO and I think it works very well. Just for the heck of it, I replaced it one day with a retail scanner antenna and in the end I thought my home built antenna's reception sounded much better.
This will usually be the case with wideband antennas vs. narrowband antennas. There are other variables, but in general you are better off with a resonant antenna than you will be with a broadband antenna (e..g, discone). A discone is good if you need to cover a wide frequency band. For example, if you wanted to cover VHF (civilian air) and UHF (mil-air) with one antenna, the discone is a good choice.
More important than anything, though, is getting the antenna as high and as clear of obstacles as possible.
Dave