Pool companies will badmouth each other because they are each other's competition.
My recommendation:
1- Check each company out on the Better Business Bureau. Make sure that if there are complaints, that these have been resolved to the customers' satisfaction.
and
2- Ask around: Neighbours, co-workers, family, friends who own a pool... and find out if they were happy with the company they selected. There's nothing more valid than the customers' perspective on whether a company delivered on the goods.