If you're ready to do a PhD now, take the offer at UIUC. There's no guarantee that you'd have more luck next year, and UIUC is a fantastic school for materials science!!
Even if you don't think the research fit is perfect, most people grow to love their grad research even if it isn't what they initially thought they'd be doing. There were reasons why you chose to apply to UIUC in the first place, think back to those reasons, and try to get back on that train.
Another thought to entertain is that even if you were admitted to some of your other picks, your opinion of each university may have shifted after your visitations (my opinion greatly shifted throughout this process, and I ended up choosing to go to the school that I did not think I'd end up at). So even though you think you'd prefer school X over UIUC, this may not be true, and end up choosing UIUC next year, you'll feel silly for waiting.
Also, if you attend UIUC and later decide that you are truly absolutely unhappy there, it is possible to transfer. And I'd argue that if you go through this process with respect for the program, you'd be able to do this without hurting too many feelings. If you're not a good fit for the school, it'd probably be apparent to not only you, but also your advisor, and your department. In that case, I think they'd support your decision to transfer (but only after a major attempt to help you stay). Though I truly think that if you give UIUC an honest chance, that everything would workout for you.
Good luck in your decision!!