I apologize for the extremely short diary, but it really needs no further explanation than the following chart.
The X axis has several different income levels, starting at someone working full-time for the current Federal minimum wage ($7.25 an hour) and ending at someone earning $1,000,000 in taxable income in a single year.
The blue bars represent the change from the Social Security (FICA) tax being reduced to 3.1% from 6.2% (as a percentage of income).
The red bars represent the change in Federal Income taxes paid from the Bush-era tax cuts (as a percentage of income).
As you can see, the Bush tax cuts disproportionately favored wealthy people, whereas cutting the FICA does next-to-nothing for wealthy people.