There is no single correct answer. You are always allowed to file jointly if you are legally married and you agree to file jointly. Joint filing usually results in lower tax or a larger refund, because some deductions and credits are limited or disallowed when filing separately, but the only way to know for sure is to test both ways.
On top of that, you have to agree how to split any tax owed or refund (you can request that a refund be split into two bank accounts, but if there is a banking problem, the IRS could put all the refund into one account). Also, when you file jointly, both spouses take joint and several liability for all the income and claims in the tax return. That means that if there is a deficiency or error, the IRS can come after one spouse, the other, or both, as they choose. (In other words, suppose your spouse is working under the table and you sign a joint return, the IRS can come after you alone for the tax deficiency if you are the easier target.). If you file separately, you are only taking responsibility for the facts and claims you sign your name to.
