First of all in northern Illinois you're smart not to pay extra for ultra-high SEER. Your total annual operating hours of an a/c system are only probably in the ballpark of 1000, and maybe considerably less than that if you're a person who shuts it off and opens windows when it's nice out. So, paying a lot extra upfront for high SEER will not save you much money on power. Given that, both of these systems are appropriate choices in the area of efficiency.

As for the size, since it's an existing home and you live there, it's an easy question to answer: Does your current unit do a good job of keeping your house cool on the hottest days, or are you having to run fans and stuff just to stay comfortable? If the answer is that your current unit does a good job, I'd stick with the same size you have now. (Have one of the people giving you estimates tell you the size of your current unit if you don't know, or you can go out on the unit and look for a metal plate or a sticker showing the size. If it's listed in BTU instead of tons, 1 ton = 12000 BTU).

If your current unit isn't really keeping up on the hot days, go with the one that's half a ton bigger.

All of this assumes the current one is working right. If it's been kind of on its last leg for the past couple of years, think back to how it worked when it worked right to determine if it's big enough or if you should go bigger.