well i think it really depends on the mousse/gel and what type of style you are going for.
i like mousse for two different situations. one - it gives fine hair a lot of volume when blowdried or set with rollers. (by itself, i don't really feel it does that much for fine hair)
i also think, when used sparingly, it can be really great to tame frizzy curly hair. but i still prefer a light pomade or creme wax for this, because it gives a softer appearance. (no shellac-ed look)
gel is also great for giving volume. frequently, if i am blowdrying someone with very fine hair, i put some just at the roots..then spray with additional hairspray (both done while wet). it gives great hold and allows me to get a lot of volume out of otherwise lackluster hair.
if gel is cut with water, or even a bit of pomade/shine serum, i think it is great for curly hair too..again, without looking too miami vice.
I don't know the difference but I use both. I use mousse when I'm looking for volume and I plan on blow drying my hair (rare occurrence). I use spray gel as a styling tool to keep crazy hair in place, make a curl stay, etc. The consistency is definitely different. Gel is stickier and thicker than mousse, ime.
I use mousse for volume when I blowdry. Gel I think of as more of a holding product - but I like my hair to be soft and natural, so I tend to prefer pomade to things like hairspray and gel.