Mayor's a step up from Congress, however, Garcia will probably lose to Daley. Why risk a guarantee to run a second underdog mayoral race? Especially knowing that Congress gives him a better shot at a job outside of Illinois since there's a long line of people who want to run statewide.
You're not "risking a guarantee." He will only resign the Congressional seat if he wins. If not, he just stays in Congress.
I was operating under the assumption he would drop out of the house race when/if he announced.