1) TIPS pay more. You can hold an i-bond for 30 years and get 1.1% real, or you can buy a 20-year TIPS paying about 2% real right now.
2) i-bonds are sort of 5-year TIPS, but if you're an ER, you probably want an inflation hedge for more than 5 years.
3) Don't sweat the taxes. Unless you're using i-bonds for educational expenses, you'll have to pay taxes on the interest. For both TIPS and i-bonds, you don't pay state income tax. (Consider this before you blindly stick TIPS in your IRA, for which you'll probably pay ordinary income tax -- both state and fed -- for withdrawls).
4) TIPS can give you capital appreciation. I-bonds can't. Of course, this is a two-edged sword, so the value of TIPS can go down as well as up.