Tough questions. Irs then google. If irs goes after you it could takes years to get out of trouble. If google goes after you your done with so it is instant but not painless.
I'm not afraid of google, they always send nice emails with recent Adsense updates etc etc. If you don't do any fraud, you shouldn't fear google emails.
Letter from the IRS - not all emails from Google are bad news (just most) whereas the IRS only write if they want money!
IRS. My dad got in trouble once by accident and it took quite a bit of sh*t out of him. Plus, I'm legit so I don't have to worry.