I'd say go to a bunch and get quotes, don't decide before the quotes! haha! But honestly it's true!
Also what is great are the perks that different places give you, such as free rooms or every 6th guests is free or every 21st guest free, etc! Check it all out, some prices per person may be great but their booking process not so fantastic...for us Westjet had great prices but was not a great option due to the fact that they wanted us or one person to manage the group and all deposits being made and such, and I was not willing to be responsible for collecting credit card numbers, money etc!
Romantic Planet didn't even respond to my initial email request for over a week and I wasn't even at the point of asking for quotes yet so I just left them as I knew I wanted a company that was on top of requests and didn't take forever to respond. Then again I have a friend who booked with them and has had no issues.
I guess it depends on your size of group and other factors as well, but I would say you should definitely do some shopping around, it can't hurt!