It's not clear as to why you went with the smaller wheel, is it only price? Of course it's going to cost less for them cause you can use your stock tires, plus the wheels cost less but...
Looks, just about everybody thinks larger wheels look better but..and this is critical...that's not all. Stagger is part of the agressive look (wider on the rear).. Some like them over 20" but not me, that is the max I'd go with on any budget.
If your in a pothole invested area the smaller wheels have an advantage in that the treadwall of the tire would be taller on smaller rims, thus less prone to damage.
Your not changing your gear ratio by changing the wheel size unless you go off the recommend tire size for this car. For example a 245/55/18 (on the LS) is exactly the same as 245/45/20 / 275/40/20 (SS front/rear). There is a couple charts which are online I used for this, it shows how much under or over diameter/price/how much it effects the speedometer, etc.
Once those babies are driven off the lot, and or maybe after they are mounted they are yours and you'd have to deal with the manager if your not happy so do any exchanges or returns at that time or best yet before so. (Don't ask how I know). They might just charge you shipping cost and maybe labor for exchanging them, if your lucky.
I think, and this is not common, that you could go with 1" larger on the rear, like 18" front, 19" rear or 19" front, 20" rear. This is an idea I got from looking at a Shelby Mustang I saw, not sure if this was the stock set-up but it is stock on the legendary Ford GT40. So whatever the reason they did that, I don't know, but it sure was not the price
The taller the treadwall, the more protection you'd have against pothole or curb damage, plus weight and price.
Stagger costs you more because they can not be rotated, but that is not to say you can't get a set that is wider on all 4 (275 or evan 285).
I'd like to see more pics of 19"s, seems the jump is allways from 18" to 20"'s or 20"s to 22". I'm not sure how much 1" would effect the looks but I know the LT2 with the 19" wheels looks putzy next to an SS with it's 20"s.