I would go for A too. That way, the kids get two weekends of excitement - one with GPs and the next one with guinea pigs, rather than having it all at once. The guinea pigs will be something for them to look forward to after the GPs have gone.
Plus you can use the GP's visit as an excuse for not getting the guinea pigs this weekend and that way you can easily save the money by getting the hutch on ebay without the stress of the kids being disappointed! Bargain!!