Mousse is best for adding volume to a hairstyle. Gel is for adding definition and hold. Those are the basics. There are also mousse-like gel products that volumize and define. Mousse can also be combined with gel to get the benefits of both.

NaturallyCurly Editors Review Target's Fall Beauty Faves
mousse kind of makes my hair feel all crunchy :/ i like using gel & spray gel & a little bit of hairspray :D
Use gel and mousse together! For my "regular hair "cocktail (normal hair: 3c/4a; BKT 3a/3b), I dry my hair a little with paper towels (even better for my hair than t-shirts!!). I do a leave-in conditioner type of something (I like CD Hair Milk), then I will section out my hair and comb a gel through (Dont' really have a fave gel, unfortunately), then I flip my head over and put a ton of mousse in my hand (Have NEVER found a better mousse than Joico JoiWhip - have been using for about 12 years)then coat my hair with it. I'll scrunch with paper towels (and they are reusable, I just hang dry them after I'm done) some more, and then diffuse w/ the Diva Diffuser, or let it it air dry. Basically, you don't have to choose b/t one or the other. I have massive curls so I like the control of the gel, but the mousse kinda makes my hair less crunchy and less frizzy. I can get away with gel and no mousse if I'm out, but my normal curls will not do anything but frizz if I do not use a gel. Try cocktailing the 2, but BE SURE to SCRUNCH after using the mousse, or your curls will get kinda limp.