When I was pricing up for a package in August, booking with Disney direct was the cheapest. The prices varied quite considerably, it was either Paris Travel Service or Thomas Cook which was the dearest (can't remember which one now) coming in at about £300 more than Disney. In the end we bought annual passes and booked the Explorers seperately with Expedia through the Eurostar site and saved ££££££££££. They were the cheapest of the hotel booking sites for our dates.
We've been discussing this topic recently on the UK board and I think sometimes packages work out cheaper, sometimes it is cheaper to DIY. I think it all depends on which hotel you want to stay at, how many children/adults are in your party, how long you want to stay and when you are going. Although it worked out cheaper for us to DIY to DLP, it is much cheaper for us to do a Virgin package to WDW because my elder DD's are classed as teenagers and not adults, therefore saving me lots of ££££.
In a nutshell, once you have decided on the dates you want to go, just get as many quotes as you can.