Age-old adage - "Use the right tool for the right job."
The longer and shorter ones are for two different types of "parties." The longer one is simply more formal and the shorter one is more casual/playful. More formal occasions will demand the longer dress and if you wear the short one you'll only end up looking like a slut. But less formal occasions would certainly be more fitting for the shorter dress.
Think of it this way. If people will be drinking from wine glasses, the longer one will be best. If people will be drinking beers, shots, or mixed drinks, the shorter.
if your going to homecoming. either long or short works, if its prom, better to stick with long as prom is a little more sophisticated than homecoming. if your talking about any normal party, short is better. UNLESS. its for some extravagant party like a wedding dinner or such, better to go with longer ones there, don't want to outshine the bride :D
depends on the occasion. I like the longer ones better but not because they are long, just because I don't really like the other two that much. Generally not my style (im more into classical glamour, retro, rockabilly, pin up , and indie clothing) but if you like them then wear them! ignore the idiots saying short dresses are slutty. this is the 21st century you don't have to cover up all the time.
if its a less formal party you may feel out of place in the longer ones though.
I think yours are fine, though maybe a little unoriginal. And long dresses, to me, are best for special occasions, like grad or prom or something that holds a lot of importance, that being said a short dress will also be right, unless it's a really formal ball. The second dress your mom likes has no shape, but the first she likes is really pretty.
If you want everything hanging out, wear yours. I like your mom's choices, but if you want to be more like everyone else, get something in between. You can look amazing without dressing like a slut. My favorite dresses fall a few inches above my knees or almost to my knees, and I get a lot of compliments when I wear them. Decent people don't want to see your ass hanging out lol.
I think it depends on the event. Some events--like a club party or an informal party with your friends--the shorter dresses are better. Others--like prom and formal parties---it is better to wear the longer ones. It just depends on the situation and what the climate is like.
I'd go with the first dress which your mom has chosen.. the sequined silver one.. It's not so formal yet very flattering. All you need are nice heels and dangling silver earrings.. and you're all set to impress ;)