What are the advantages or disadvantages of joint tax filing (husband and wife) over filing individually?

We both earn income from employment. Our planned deductions are donations, medical expenses and public transportation cost.

Well first being legally married you are legally suppose to file as so. As a single person your standard deduction is 5K + and as a married couple it is doubles to allow for the additional income. For you to claim your medical you will have to pass not only the standard deduction cost but you will have pass the minimum medical allowed. Ex if you and your wife make 100k then you will have had more than 7,500 in medical cost before even $1 counts toward your standard deduction. Your donations will help to reach your standard deduction. Usually people without a home do not exceed their standard deduction. Now public transportation? are you talking about commuting to and from work, because that is not deductible (everyone has to get to and from work) Unless you do some sort of sales and your job requires you to travel and does not reimburse you for any of those cost. Depending on how much you and your wife make its not always the best scenario as far a your taxes, but since its the only way you have to go it makes no sense to make you mad and tell you its better as a single person.

The answer above (yet again) refers to US law, so ignore it.

In Canada, you each file separately, but indicate your marital status on the form. It is not optional, you have to indicate married if you are.

The only real difference is that for certain benefits (like GST credit, for instance), household income is used.

Medical expenses are only deductible over 3% of your income, btw.

But as I say, there is no choice to be made, if you are married or common law (living together) you have to indicate that, or risk being charged and fined.

The post above me "consultant" has no clue what he is talking about.

Nobody files jointly in Canada. Each person files individually. Donations are usually claimed by the higher income spouse (to reduce provincial surtaxes), medical on the lower income spouse.