Unless your mother's income is sufficient on her own, she would not be a qualified "joint sponsor". Do you have somebody else in mind? If you will be working full time "shortly" then that means you've changed your plan to stay in the D.R. for the duration. You working solves the whole problem, as you will qualify as sponsor on your own. Yes, the CR1 would be a better option, as you on not submit the affidavit of support until a year or so after filing the I-130 petition, and your wife will be immediately eligible to work, once she arrives.