Re: Why go from a 17
Only reason I can think of to go from the 17" to the 18" wheel is because either 1) you like the looks of certain 18" wheels, and/or 2) you need the 1/2" more room to handle larger rotors/calipers.
My personal favorite stock Viper wheel from a looks standpoint is the 18" "wide-five-spoke" wheel they start putting on in '98 or '99. I wanted to buy a set of these for my '95 RT when they came out, but spending around $4500 (wheels AND tires) for just looks didn't fit into my budget. I was able to finally get them with my '00 RT.
HOWEVER, from what I understand, the "wide-five-spoke" 18" stock Viper wheel offers significantly less brake cooling than the old 17" "narrow-five-spoke" and GenI three-spoke wheels, which is really only an issue on the track. And, if you have an '01 or '02 with ABS, you can't use a 17" wheel anyway without shaving at least one bolt off, which probably isn't an option.
I use a set of GenII 17" "narrow-five-spoke" wheels for my track tires, partly because 1) I had them already, 2) the tires are cheaper than their 18" models, and 3) because of the aforementioned brake cooling issue.