You're changing wheel size anyways, so, here's what you're faced with:
1) what look do you want; and connected...
2) how do you want to drive your car?
Here's why I start there. If you want a wide tire look, you really need an 8" wide rim just to go 235 tread width if you want to maintain a stiff and flat sidewall.
Basically, you *can* fit wider tires than that on an 8" rim, but, you start to see a sidewall that bulges outwards in the middle. A stiff sidewall looks more like [_] from the side. As you go wider and wider it becomes (_). This changes your handling characteristics as it flexes.
Your second piece of the puzzle is how you arrive at wheel choice. Are you upgrading brakes? Are you doing it to improve tire selection? Do you plan on sticking to stock GM wheels of the chosen size, or, going aftermarket anyways?
For brakes, not all 15" wheels will clear. On a given upgrade, some wheels work at 15, some don't, due to backspacing and caliper interference on the inside due to how the wheel face attaches to the barrel. 16" are generally safe.
One thing to note, now many 15" tire options are beginning to disappear, just like 14" had. I'd think a 17" wheel actually gives you the biggest selection of brands and treads for a really good window of time, *if* you're buying aftermarket rims, and, sont mind a thinner sidewall/harsher ride quality.
There are good things about the 14' and 15" sizes - taller sidewalls protect the rims better in pothole prone areas or rougher/unpaved roads. They also flex more to give a smoother ride, at the expense of a more floaty feel and not as stiff of a cornering feel.
Really you need to think about how you feel about those aspects before narrowing down what you are in the market for, and, get into questions like whether you intend to update to a shorter stiffer spring pack which will slightly lower the car as a side effect due to how they're made.