It depends on a lot of variables. If she is having any elements inside the parks, she doesn't get a choice. She will have to use DPS. A couple of years ago (when we got married) DPS didn't have a website to see samples of the different photographers work, and many of the photographers had limited skills it appeared, as many couples were unhappy with their photos. That is when a lot of us sought outside photographers.
In the past couple years, their products have seemed to improve. We did an MK and AK shoot and were very pleased with both DPS photographers we had (Ty, who is still there, and Paul, who has since left DPS). I know there are several newer staff that have done really nice shoots lately, so I don't think you will be disappointed with their services.
Since we had heard a lot of negatives around the time of our wedding, we looked at outside photographers for the actually ceremony. I did a lot of research and decided on Randy. We were beyond happy with him and his products. Please note, the photo below was not his, so don't judge his work on that one! My DH and I both LOVE Randy and I hope to be able to hire him again in the future !
The biggest issue with DPS -vs- outside photographers at this point is the expense. Most outside vendors give you a pretty good deal that includes the high res CD with all your images and copyright. That means you can do what you want with them and print them where ever you want, fairly inexpensively. DPS is still quite costly for the CD and to order prints through them is still pretty expensive too.
Most importantly, research the photographers styles and choose the one that produces images in line with your taste. Everyone has different ideas of what they like and luckily there are plenty of vendors out their that do things differently. You can get into a huge debate on here over who is "better", but it really boils down to what style you like best.
I am sure many more brides will come along with more info.