Two considerations: other musicians you might like to (eventually) play with, and your own singing voice (if you now or someday might like to sing with your playing).
For other instruments in the Cajun genre, C or a D is a good choice. Fiddlers like the keys of D, A and G, which are the keys you would play on a D accordion. But Cajun fiddlers often tune their instruments down a whole step so they can play in C, G and F, the keys you would play on a C accordion. Guitarists can do it all.
Most men, in my experience, find that Cajun songs on the C accordion work best for singing.
C is the most common first Cajun accordion choice, and it is what people expect you to play at jams.
If you're going to be learning off of instructional dvd's, I think you should go with a C. All instructional's I know of use a C accordion, and most recordings.
In preference, I'd rather a D. But for someone learning Cajun music, especially off of dvd's, and if the potential is there to go to jams and instructional camps, you need a C. Once you get good at taking a tune and converting it from your ears to your fingers, it doesn't matter what key except for what you sing in, if you sing, or who you will play with, if you will play with someone els.