I think ducks are quieter and less obnoxious-sounding than roosters. But the duck hens are louder than chicken hens. On the other hand, if she's worried about neighbors, most people really don't mind the sound of ducks--they actually tend to like it. Whereas a lot of people will complain about a rooster. For one thing, roosters tend to be really really loud right when people are trying to eke out an extra hour of sleep in the morning, whereas ducks will generally wait to make their noise until whatever time they are accustomed to being fed (they are not super-early risers as a rule).
They do poop a lot. I have noticed that the babies are WAY messier than chicken babies. I only have to change my chicken brooder every couple or three days, and then I only have to remove the top layer of bedding, not the whole thing. The duck brooders always have to be changed twice a day, completely, and rinsed out at least once a day. They are pretty gross. And they get everything wet and dig mud puddles in the lawn wherever they can find water to moisten the dirt with.
Mine do tend to lay eggs wherever the heck they happen to be when they get the urge. However, they also tend to lay at night, so if you keep them enclosed at night (and you should--because otherwise you *will* eventually lose most or all of them to a predator), you will be able to find the eggs without too much trouble. On the other hand, the eggs will be dirtier than chicken eggs. They will also, however, lay more eggs, larger eggs, and more nutritious eggs. And they use less feed per pound of eggs.
I don't think they need a lot more space, although as someone has already said, they are larger so will need slightly more.
I completely understand your mom's wariness, especially if she's had both before and already knows her preference from experience. But there are some advantages of ducks:
They are nicer than chickens. Nicer to each other, and nicer to people. I've never heard of a drake attacking a human the way a rooster will sometimes. And they don't pick on each other at all, usually (there are exceptions, mostly to do with management--if you have too many drakes for the number of hens, there will be fighting, for example).
I think they're more fun to watch than chickens. I mean, chickens are cool, too--don't get me wrong. But ducks are just so amusing and cute!
They don't have to have a pond (and the others are right about the dangers of crossing the street to the pond--and you will have to teach them where it is and that it is safe to use--this is not always as easy at it seems like it ought to be), but they will appreciate a little kiddie pool to get wet in.
They're awfully cute.
They have simpler needs--no coop is necessary--just a predator-proofed enclosure and a dog house or other structure for the coldest of nights.
They don't suffer as many ailments. Ducks run a feverishly high temperature as their normal temp, meaning that not much in the way of bacteria or viruses can survive in their systems for long. So they just don't get sick as often. They do, of course, have things they can suffer from, just not as much.
They are more weather-proof than chickens--no worries about them getting wet in the rain, for instance (they LOVE the rain), and they can handle colder temps too without any trouble whatsoever.
I do like them a lot. But chickens are way cool too. I sometimes think I should have done the chicken thing because they are easier in some ways, and neat in their own way. But I love my ducks, and plan to keep messing around with them. The ducklings are sturdier & hardier than chicks. And cuter (sorry, but it's true!).
If you do a search on this forum for "100 Things I love about ducks" or something to that effect, you'll find a thread that gives lots of other cool things about ducks that you could perhaps show your mom, if you still want ducks.
But either way, you will have a blast. Poultry is fun.
