I hadn't thought of the singing question. I have played the harmonica for a good while and Bd is my favorite key in that. More minor notes and pitched lower, which may make blues much easier (another of my interests) Many good reasons for each choice, it's a personal thing. Right now the Bd seems to be in the lead.
I say D and then Bb after. Here are some other factors to consider. Where do you live? Who do you think you might be jamming with? Most fiddle players are tuned standard (GDAE), not tuned down one tone (FCGD)like in SW Louisiana. It is much easier for fiddler to jam with you if you're playing a key of D box. For a key of C box, many fiddlers will prefer to tune down to capture the effect of the drone strings which make the cajun sound unique. From what I know, the Bb box is more typically associated with Blues and Zydeco music. Again, you singing tone should also be a factor.
Maz
Here in Austin there are not that many Cajun musicians. Zydeco is much more common. Of course, there are a whole mess of Bluegrass/Irish/Folk. Maybe I should just get a three row. No hurry for now. I am doing fine with my C box