Generally, tires for 18" wheels are more expensive than tires for 17" wheels. Keep in mind that you'll have to buy four tires at a time, so that's a pretty good chunk of change right there if anything should happen and one of your tires gets destroyed for any reason (sidewall damage from driving on a flat, blowout, etc.).
Also, 18" wheels tend to bend easier due to potholes and other road debris that are sometimes unavoidable. Replacing wheels can become expensive too.
It's sort of unfair to use a blanket statement to describe the weight of 17" wheels vs. 18" wheels. True, a 17" wheel in the exact same style as an 18" wheel will be lighter, but there are certain 18" wheels that can be lighter than some 17" wheels. Wheel weight is definitely something to take into consideration when buying any wheel though.
Looks-wise, I prefer the 18" wheels to the 17" wheels in most cases.
You'll also feel more bumps with an 18" wheel than a 17" wheel.
As far as which one is more "responsive," there are other factors involved than just tire sidewall height. I'd wager that ultra high performance sticky summer tires mounted on 17" wheels would outperform a set of all season or otherwise crappy quality tires mounted on 18" wheels.
So basically, if cost and ride comfort are important to you, buy a set of 17" wheels. If you have extra cash to blow and want to look good, go for the 18" wheels. It's really your decision.
If I were you, I would spend the majority of my money on tires and use whatever is left on wheels. The tires should definitely be a priority.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)