The stock antenna is very much a compromise - in fact, it's likely lossy at many frequencies, and not much better than a wet noodle. In addition, you must consider the attenuation of being surrounded by metal inside a vehicle.
You didn't say anything about what freqs you are interested in hearing, so it's not possible to say for sure whether the RS magmount will work well - however, there are MANY other alternatives in our scanner antennas wiki, and there has been some discussion just recently about yet another mobile antenna (similar to the PMM4B Valor model, or so it says) that supposedly works well.
Don't just read the new posts, Frank. Sometimes checking the old posts will give you lots more information 73 Mike