I believe wheelchair users should definitely get priority on buses. They have a lot less mobility than people with prams. It is more of an inconvenience than anything for people to fold up or move their pram. For Wheelchair users it is a seriously painful struggle or they may even be physically incapable of moving out of their wheelchair by themselves.
Of all the time I have been catching buses I have never came accross this problem though. On the buses I use there are two dedicated spaces that can be used for wheelchairs users or people with prams. Bus companies would be better off figuring out how they could better utilize the space on their buses rather than getting involved in a wheelchair vs pram debate.