What s the advantage to being W9 vs. just being paid under the table & just reporting that earned amount when I file taxes?

The advantage is that you'll have the paperwork to show how much you made. This is important for two reasons. One is that if the IRS doesn't believe you and thinks you reported the wrong amount, you may need to show them documentation of what you really made. The other is that if you apply for any loan or to rent some homes, you need to show proof of how much you make.

You are required to provide your clients and customers the information requested on Form W-9 upon their request. If you fail to do so, they are REQUIRED to withhold 28% of your invoiced amount and turn that over to the IRS and pay you whatever is left.

Smart businesses simply refuse to pay their subs and service providers until they receive the completed Form W-9. Not getting paid is a pretty strong incentive to comply with the law.

the only advantage is that you are considered an independent contractor, however you may find that the conditions you will work do not actually comply with the 'independence' this status implies

as self employed you pay your own FICA taxes, you are not eligible for unemployment, you are not covered by workmen's comp, it is up to you for your own health insurance, if you want a day off or vacation you don't get paid for it, et al

If you do the work and then refuse to give the payer a W-9, the payer is REQUIRED to withhold 28%. They will attempt to mail you a 1099-Misc at the end of the year (assuming they've got your correct address).

There is one possible advantage:

The person paying you is required to issue you a 1099-misc if they pay you more than $600 total for the year. If they don't have your SS# from a W9 form they will issue the 1099 with the tax ID field blank. That can cause 2 problems for you:

1. The IRS could question whether or not the income is really yours. Its unlikely that they would, because they generally don't object to people claiming MORE income than reported. But its still the kind of thing that could draw extra attention to your tax return.

2. The IRS could issue a required back-up withholding. This would require the person to withhold 28% of your pay toward taxes, effectively forcing you to claim it if you want the money back at tax time. It still works out the same in the end if you're planning to report everything properly, but it takes control of your money away from you.

The payer (your "employer") is the party that "needs" and will require the W-9. In such a case, this is self-employment income.

NO advantages mainly because you will need to keep track of, and send in, the proper amounts for all the usual deductions found in a paycheck