Usually, in most cases, generally speaking, most of the time, for most people, on average, and assuming there are no issues with your cabling or connectors, you will get better send and receive performance using an external antenna vs using the HT antenna INSIDE the car. I would expect the external antenna to work a little better than the stock antenna outside the car, but not hugely.
Dont forget that you are losing some power through the coax, connectors and patch-cord, so that could negatively impact the performance.