There is a difference between hair gel and mousse as anyone will tell you that have used both and I use both, on myself, as well as my hair clients. I am a hairstylist and have been since 1983, so I know from first hand experience, and a lot of it, the difference between hair gel and mousse. Just by looking at both hair gel and mousse you can see a difference and that difference is weight. Hair gel is heavier and is designed to provide a stronger hold and also to manage and control frizzy hair. However, when hair is excessively frizzy other types of anti-frizz styling aids and conditioners may be needed to achieve the finished look desired.
Hair mousse is fluffy and light and typically has conditioning agents; this is why it feels silky to the touch and not as sticky as hair gel. Hair mousse is used more by those with fine hair to get volume and hold. Just like hair gel, mousse comes in a variety of hold levels such as strong hold, extra strong hold etc. The stronger the hold the less you need to use on the hair or hair may become crunchy or brittle and as a result cause hair breakage. If this happens first use a pick then a brush to prevent hair from breaking and proceed with styling hair as normal. Forcing a brush or comb through hair also tends to make it frizzy.
If you have already bought hair gel and have found that it is too heavy don’t discard it or give it away simply dilute it by mixing it with water in the hair. You do this by leaving hair wetter than normal after shampooing and then applying the gel. The extra water in the hair will dilute the gel, making it weaker, similar to hair mousse minus the conditioning agents. To make this work effectively you also need to comb the product through hair to ensure complete and even coverage. You can do this with hair mousse as well if you find that the hold level being used is too strong. This method can also be used to conserve usage of hair gel and mousse.
The difference between hair gel and mousse may be significant to one person, but not so much for someone else. This is due primarily to hair texture and styling need and it may take some trial and error to get it down pat. There are other factors to consider as well such as weather, fresh color or perm or freshly cut hair vs grown out hair that requires more hold or styling. As you can see there is more involved than just the hair product.
Also see Best Hair Products for Scrunching Hair