When I lived in an area where simulcast 700/800 MHz traffic was the dominant band, I used a Diamond duck on my SDS100. Now that I am in an area where VHF abounds, I have an old Antenna Specialists magnetic mount VHF antenna mounted on top of a large safe. Living on a hill also helps reception.
Assess where you live, what traffic you want to monitor, the amount of space you have for antennae, the amount of distance between the antenna and radio - knowing that > distance + expensive LL coax = minimal signal loss, and how much cash you have/gear you can apply to your situation.