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

Rick Reid said: "Bb. Might as well go with something a little further away from the C. D is only one key up from the C."

Uh, unless I'm missing something, Bb and D are exactly the same distance from C, namely two semitones: Bb - B - C or C - C# - D, it's the same.

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

Keystones were kickin' in. You are correct, sir.

R!CK