The ASP-7455
One suggested mobile antenna is really a COMPROMISE.  Listed below is the Antenna Specialists ASP-7455 150MHz 3db antenna.  Wait… isn’t the 6 meter band on the 50-54MHz frequency spectrum?  Why yes it is… While listed as a commercial 3db gain antenna for 2 meters, the ASP-7455 will work as a 1/4 wave (unity gain) antenna for 6 meters.  This would allow the user to have ONE antenna for 2 bands.  Try tuning the antenna to the TRANSMIT frequencies you want to use the most.  (This might be around 52MHz on 6m, and around 146Mhz on 2m. )