if your commissions on included on your W-2 (which they should be if from same employer) the tax rate is the same.
if you earn commissions not included on a w-2, you would file schedule C. the regular income taxes are the same. but in addition if net schedule C income is over $400 you would pay up to about 15% in self employment tax in addition to the regular income taxes.
but again if they are on the W-2 , it makes no difference whether it's salary or commissions.
