You're almost always better off married filing jointly (MFJ). Many tax benefits aren't available if you file separate returns. Ex: You can’t claim a majority of the credits if you file separately. Also, if you file jointly, your standard deduction (if you don't itemize) will be higher. This usually causes your taxable income and tax to be lower.
Married filing separately (MFS) might benefit you if you’re liable for Alternative Minimum Tax (AMT) on a joint return. However, this is only true if only one spouse is liable on a separate return.
The best way to figure out whether married filing jointly or married filing separately will benefit you the most is to prepare your returns both ways. Then, choose the filing status with the lowest net balance due or refund.
If you choose married filing jointly, both of you can be held responsible for the tax and any interest or penalty due. One spouse might be held responsible for all the tax due -- even if the other spouse earned all the income. If either spouse doesn't agree to file jointly, then both spouses must file separately. There’s an exception if one of you qualifies for head of household status (HOH).