You cannot book with one TA and then switch to another under normal circumstances. You can rarely, but the process is quite involved--the first must turn the reservation back to DCL (agreeing to lose their commission). In most cases, a second TA won't touch it after that.
What you can do is to book thru DCL and then transfer to a TA. The disadvantage of this is that the TA receives a lesser commission for a transfer than they get for a booking. Some TAs will not accept any transfers, others give a reduced commission for a transfer.
ALL TAs must sell Disney cruises for the same price if you are comparing "apples to apples." Where things may SEEM to differ on web sites, etc. is that some TAs do not list taxes in the numbers on their sites, others do. If you get a quote on the DCL site, you will have to remove the transfers and insurance to get the basic cruise cost.
The only difference you will see among TAs is what "incentive" or "gift" you will get from them for booking. What is less easy to determine is how good their service is or what you can expect from them. In my opinion, you should be shopping for the service and consider the onboard credit a bonus. After all, if you are sitting at an airport with a flight canceled and need to get to Orlando, do you want to be able to TALK to a LIVE person or do you want to be waiting for an e-mail to be returned? Do you want to talk to an agent who is totally on commission (so doesn't really care if he/she didn't do YOUR booking?)
Example--when the Magic couldn't leave
Castaway Cay a week ago there were long lines at the GS desk of people being placed on "permanent hold" by airlines, trying to work out new flights, etc. If I had been on that cruise, I would have needed one phone call ...or I could have chosen an e-mail...to my TA. The agency does a huge number of DCL bookings and likely would have already known of the situation, but if not, I would have advised them that I was on the Magic and we would be getting in a day late (or whatever DCL had told us)...and I needed them to work out the travel back home. Id be able to talk to a live person because they have "extended hours" when their office is open as well as an emergency contact number. The person I'd speak to would be on salary, not commission--every agent in the office is MY agent. I'd probably have told them that I'd call back when I was at Port Canaveral to see what had been worked out and trust them to deal with it. When I got back to PC, I'd have flights booked and probably they would have already pulled some of the stuff to start a claim with the travel insurance company.
Does all this sound impossible--no, it's not. It's called service. Can it happen? Well, I was once notified on Friday late afternoon that the Magic would not be sailing the next day and I had certain options.....and I was already at WDW so I'd need hotel, etc. I was once at LAX, having been told to meet the DCL rep and there was NO DCL person anywhere. The Delta people kept saying "but they are always here....and paged them...but I got my TA on the phone and said , "What do I do now?" They got DCL on the phone and got me where I needed to be.
My point--shop for your TA before you book. Get IN WRITING what you will pay and get if you book with them and if it will be the same if you book thru DCL and transfer to them. I don't think transferring is generally a good idea, but you might. But look at WAY more than how much OBC or gift card money you might get.