I am going to be retiring early and will be getting health insurance through Marketplace. My husband and I prefer to file separately which means I don't qualify for the Marketplace premium tax credit. Is there a significant tax benefit to us filing jointly so I will qualify for the premium tax credit? Or will it work out about the same if I itemize deductions at the end of the year? I won't have much, if any, income during my retirement so I'd like to know which is a better approach. Thanks.
Yes, the significant tax benefit to filing jointly is that you will qualify for the Advanced Premium Tax Credit.
If you file separately, you will not qualify for the Premium Tax Credit and if one of you itemizes your deductions, both of you will have to even if it's not beneficial for one of you to do so.
It is almost always better to file jointly.
Here is a great article explaining the advantages of filing jointly and the consequences of filing separately. Filing Jointly vs Separately