They both work well and you probably can't tell one from the other when monitoring.
The difference are that the Diamond are flexible and use a matching coil that reduce the lenght 5 inches compared to your telescopic antenna when extracted to the correct lengh of 19-20 inches to handle both 150MHz and triple that frequency at 450MHz.
It's a matter of convenience that you do not need a sma-bnc adapter and the antenna can flex, but performance are equal. The telescope can be adjusted to cover 75MHz to 1000Mhz so it handles more frequency ranges like aircraft 118-136MHz at an antenna lenght of 23 inches. If you are concerned that the telescope might catch to something and bend or break then the flexible Diamond are a better choise for those 150Mhz and 400MHz frequency ranges.
/Ubbe