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Are Wheel spacers the way to go on my 2007 Si, to fit wider tires and rims. O do I go with a wider rim with a different offset

 
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Spacers will give the appearance of wider wheels and tires but will just widen your stance. Spacers are cheaper so its a cheap way to have a widen stance. Spacers can be used to get that extra bit needed to flush the wheels.

Wider wheels with lower offset would be the only way to get wider wheels.

 
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What are you trying to accomplish?

Spacers (or adapters) will push the wheels "out", so they'll be positioned closer to flush with the fender flare. It'll "fill out" the wheel well in appearances. But spacers obviously do nothing to widen the wheels or tires. They do widen the track of the car a bit, though. And they may change how your suspension feels, as the tires won't be located exactly where the Honda engineers originally intended.

If you want wider wheels and tires, though, the better (but more expensive) way to go is to get a wider set of wheels in the proper offset and backspacing to fit your car. That way the centerline of your new wheels will match that of the OE wheels, minimizing adverse effects on your suspension and handling. With proper offsets, you shouldn't even need spacers. Avoiding spacers will also minimize unsprung weight, thereby making your car accelerate, brake, and handle better.