I am asked this question quite often. The answer depends on what you want from the reed. There are quite a few differences between a molded reed (an Apps G3) and a ridge-cut reed (an Apps G4). Here are my observations
The G4 reed can be described as having a strong bright sound which really blasts from the chanter. It lacks some of the subtly and depth of the G3 and doesn't present as strong an overtone which some soloists look for. It does however give a pipe corp. a big presence in a pipe band helping the balance between pipes and drums. The high A is usually clear and true without any vibrato.
The G3 reed has a warmer sound and will produce more harmonics. The high A can be adjusted to produce a clear sound or to produce a vibrato/crow.
There are several ways to physically adjust both styles of reed but the most versatile is the G3. It is easy to set a G3 to the strength required but scraping and it will stay at that strength so long as it is not blown too hard or exposed to too much moisture or dryness. It is also easy to make fine adjustments to the tips of the G3 to effect the top hand (High A and High G) whereas the tips of the G4 are best left alone.
The G4 reed responds very well to a good squeeze and will come to life and free-up once this has taken place. If you applied this to a G3 you would probably ruin it!
The G4 tends to be more stable and less liable to move pitch with variations in pressure especially on the top hand. This is possibly the main reason why it is preferred by bands. Players can get a little 'excited' when entering a competition circle and this can lead to some overly zealous blowing which in turn can lead to sharp notes! This is less likely to happen with the G4.
The G3 is stable but will move a little when overblown. This is very easy to hear and correct when playing alone but much harder to do when playing in a pipe corp.
In conclusion, both reeds are great for both soloists and bands but each has its own idiosyncrasies.
For fully detailed and illustrated instructions on how to get the best from both reed styles buy my 'Complete Guide to Highland Bagpipe Reeds'. It will help get the best from your instrument and save you a lot of time, frustration and money.