Let's face it, there are some big limos out there. Some hold close to 18 people. Our bus seats 22. Not much difference until you sit in one for a while. With a bus, you can get up and stretch your legs or move to another seat with ease. No more talking across people.
Buses can be safer due to their size, weight and height. Buses are commercial vehicles designed to carry a lot of people from the factory. Most limos are highly modified vehicles that start life as a car or suv and then various small and medium customizers cut them in half and add filler panels and beams in the middle to get additional length. Now we don't have a problem with limousines really and we may get one or more ourselves someday.
If your group has 10 or more people, or you just want the extra space, why make them all crawl into a limo when they can walk onto a bus. If anyone's a bit on the claustrophobic side, they'll really appciate you.