It is much easier to collude with peers than with clients, so I'm more likely to trust reviews from clients than peers.

Much easier to lie to customers though.

When you book a hotel do you base your decision on reviews from other customers or other hotel owners?

With professionals it's their peers. Unfortunately that doesn't really help unless you're in the "club". In any community the other doctors all know who are the best surgeons and who are the butchers. But they won't publicly badmouth a peer or even privately tell you unless they really trust you.