Generally speaking, the amount of antenna you have in the air, the better the reception...provided that the antenna impedance at the feed point matches the impedance of the system...therefore...if both a long antenna and short antenna were compared and both properly matched, the longer antenna would produce the better signal. Antennas are usually matched by top loading with capacitance or bottom loading with an inductor (coil) to provide proper matching between the antenna impedance and the feedline impedance. Since this is the realm of the manufacturer, most antennas sold will have proper matching built in to them. In some cases, the last little tweaking is left up to the customer and the instructions with the antenna will say something like: tune this screw for maximum signal. Now the reason you may not notice the difference on the shorter antenna is that compared to the longer antenna, the signal drop may only be about half or about 3db. You would not notice the difference in signal on FM because the signal strength has little to do with fidelity. The only time that you would notice a difference between the two antennas would be in the fringe area of the station that you are listening to. And 3db or even 6db of signal difference between the two antennas will make little noticeable difference. At 10db, the effect will be noticeable. So basically, shorten it too much and it becomes like having almost no antenna at all.
I guess I'll keep my stock one until it breaks. Just hope it doesn't hurt too much.