If you already have a 'PO' mount and don't want to put another antenna and mount on your vehicle, just find an antenna that uses that 'PO' mount. Then it would just be removing one antenna, replacing it with the other antenna (and the same with the other end of the feed line and radios).
It's a matter of choice/preference, but I like NMO type mounts. They tend to require less room under the metal than the 'PO' mounts do, and from what I've heard and experienced, they're a bit more sturdy. Lots of variations in that, mainly a matter of just how 'big' an antenna you put on either type mount, they both have limits.
Most manufacturers make antennas that use either mount, so it's just a matter of how you wanna do it.
- 'Doc