They don't smell if they are cleaned out regularly (and others may have differing opinions about this) but I think boars tend to smell a bit more pungent than sows. Also boars have the disadvantage that they can suffer from impacted faeces when they get older which involves rather unpleasant 'massaging' input from you! (Not deliberately trying to be negative -just telling you the worst case scenario so you are forewarned!)
Boars can be more cheeky/assertive and therefore very nice to have as pets though.
Definitely buy more than one gp (herd animals). They are lovely animals for dc (if you can restrain yourselves from bothering them when they first arrive which can be very hard for young children). The first few weeks with you are vital - they need to be left alone, in a very quiet area with only gentle contact at feeding times (coaxed out with food) etc, progressing to very gentle and calm stroking. If you can give them a good start, they will be happy to interact thereafter.
All gps have very different and distinct personalities though (some more outgoing than others) and as others have said, they are quite vocal which is fun.
In terms of hygiene, I actually find rabbits easier to clean out because they poo in just one area (a large cat litter tray here) whereas gps poo all over the place. For example, I can clean out the rabbit's litter tray daily or every two days and then give their living area (half a room) a good clean out and mop once a week. With guinea pigs, their entire cage needed to be emptied and cleaned every three or four days.
However, gps are much more friendly and interactive pets for dc and can be much more easily picked up and stroked (whereas most rabbits detest being picked up). Rabbits also tend to chew everything in site which can be expensive!
As for pooing and peeing whilst walking around outside their cages etc, they do tend to do this when they are very young but get better (although not perfect) as they mature. We have wooden floors so it wasn't a disaster when it happened - I imagine carpets would be more difficult to maintain. Our gps used to kind of nod their heads up and down when we had them on our laps - and that was the sign that they needed to go back to their cage for a 'bathroom break'!
It would be great if your gps could have access to lawn during summer days - they love grazing fresh grass - but be careful to enclose them carefully as they are skilled escape artists.
Other than that you need to clip their nails regularly. Buy the famous gp care book by the eccentric Peter Gurney.
This site has some fun "tubing"/cage solutions although it can all get a bit £££££££
Oh yes, one last thing - if you are buying two - try and find a pair that already seem comfortable together in the cage.
Arf at 70isalimit and her shopping trolley - I find myself buying more salading for my rabbits than I do for myself!!
Enjoy your pigs - you will fall in love with them.
[Rabbits are lovely too for older dc and if not kept in a hutch!]