As
@CrazyRabbit noted, grants can be (and I think state grants usually are) taxable income on a federal level, because you are actually getting cash, and the IRS wants its cut. There can be specific exceptions, and apparently (I have not dug into this since I did not have any) utility rebates are one.
But the debate is actually not that big of a deal at present, because if you do not treat the grant/rebate as taxable income, you need to reduce the amount of your ITC claim. Which one is more beneficial would depend on your tax bracket and the current year's ITC credit percent, but the choice for rebates and grants is not typically between tax or no tax but between paying the income tax and getting ITC credit or paying no tax but not getting ITC credit.
And I will note that state tax credits are also handled differently than grants and rebates, and are potentially the best type of incentive because of how the tax treatment of them works.