There isn't a huge difference and PPs are right in that, if you are the type to need total control, it's definitely easier to make changes if you do it yourself. If you'll enjoy planning all the details, reading up on the best room locations for each resort you're considering, watching each week for new codes in case the price drops, sending the room requests 14 days out, ect, - and are comfortable being your own advocate if something is less than you expected - then you'll enjoy handling your own booking.
The main difference I see is that not everyone lives, eats, breathes and sleeps disney so not everyone knows the moment a new code comes out or where they want to eat & when based on park hours, nor have they visited/stayed in most, if not all of the on-property resorts. Some people don't even travel frequently given the challenges of today's hectic work schedules and uncertain economy. With so many choices, it's sometimes the tiny details or those that don't seem like they'd make a difference in the planning stages that can help to make a trip more magical. Having someone who's "been there/done that" to lend a hand can be wonderful.
Using a TA won't cost you any more - a good Disney Specialist could even save you money. It will definitely save you time. Plus, should something go wrong, TA's usually have a bit more pull when working with wdw central res/the airlines/or resort to straighten it out to your advantage.
Some agents may try to upsell in order to earn more commission. IMO that's like shooting yourself in the foot. If I can show you how to do wdw for close to the same price that you already spend for your annual vacation, chances are you'll call me to do it again sooner. If however, a wdw trip costs you five years savings, I'm not nearly as likely to hear from you again as a client.
Obviously, my goal should be to get you there as reasonably and make it as magical as possible to earn your repeat business. That's something that disney travel co and the resorts themselves used to make every effort to help the public with. Sadly, those days of wdw asking "how can we make this more magical for you today" are vanishing faster than a mickey head ice cream on a hot July day, making it even more important to do your research if you're booking it yourself.
If using someone who already has those resources, the key is to find a responsive & dedicated TA and have an idea of what you want/what you expect to spend for it when you meet them. I can't tell you how many times in the more than 25 yrs now that I've been in the office we have someone walk in the door and say "where can you send me and how much will it cost?" That's such an open ended question, our entire office could work on it all day and still not present an option the client really likes.
Some agencies are not responsive in the beginning. If you call a TA for the first time and are told they'll get back to you, be sure to pin down when. If the return call takes too long, chances are it might after you're a client too, and that's unacceptable so you'll want to contact another agency. There are lots of dedicated TAs out there who are available 24/7 for their clients. When you find one, there's nothing like it.
