We have rabbits, piggies AND rats. So I think I can safely address the smell and social questions.
Our two piggie girls are our first, so I can't compare them to boys or other piggie personalities.
We use fleece and litter boxes with wood pellets. It has taken some training, but our girls are getting very good about using their box. I'd say they were at about 80% fully trained, which I hear is pretty darn good.
My rats are also litter trained, as are the bunns. In my opinion, rats are THE cleanest, but of course, they eat more than just veggies, so their poo will smell more.
Rats are by far more social that both bunnies and piggies. We interact with all of them daily. But if my rats don't get to come out for playtime, they are like little kids that are cooped up in the house. They'll even bicker with each other, if they miss playtime one day. My bunnies have their crate door open and have free range of our kitchen. They like to interact with us, but honestly could care less about being picked up or cuddled. They are happy to have us play with their toys with them etc.
My piggies like when I talk to them and especially feed them (I was amazed how much piggies EAT)
But they too don't seem to care as much about being held, cuddled or brought out for playtime (they will hide most of the time they are out). But I think mine are just taking a little longer because they were from rescue and weren't socialized when I got them. If they don't come out, I will still stop by, talk with them, give them treats (veggies) and make sure they know I'm here. They seem to enjoy the sound of my voice and I know many folks here have piggies that love to be cuddled. I would venture to assume their piggies wouldn't be depressed if they missed a day out of their cage (as long as it's a nice big cage, like they show you how to make on this site. We started with a 2x3 w/loft. We've already moved up to a 2x4 w/loft and are building a play area with ramp off of that...so they will have lots of room).
We change the piggie's litter boxes twice a week and do a thorough cleaning once a week. Our piggies and bunns are about the same smell wize, but I'd have to say the piggies smell a tad more (only because they aren't fully litter trained yet and pee on their blankies sometimes). But the smell is more like wet hay, than a nasty dirty cage smell.
The thing I like most about the piggies, is their cute little faces, lips and the wonderful sounds they make. Rats and bunnies are very quiet. Piggies love to sing for their dinner and I still giggle everytime I hear them "wheak".
We took our piggies to the vet for exams right after we adopted them. We wanted to be sure they were in good health to start with.
I adore my bunnies, don't get me wrong. But I think they only tolerate us.
My piggies seem to like me (and getting more interested in me everyday), but I fell in love with them the day I brought them home. Just thinking about them puts a smile on my face.