normally it's a bad idea to file separate. there are a number of deductions you lose if you file separate.
The standard deduction is more valuable when you file together. that is because your half of the standard deduction saves you 12% of your income and his half saves him 22% if his income. but if it is joint, then the TOTAL standard deduction saves both of you 22% of HIS income which is a bigger savings.
you can try to figure it out both ways, but 96% of married couples file married-joint. The rules are just set up to motivate the behavior that way and it has to be an unusual situation that compels separate filing
