Which ERTL version are you talking about specifically? Is it just a regular American Muscle version? Is it AutoWorld, Route 66, Peachstate, Racing Champs, or another version i'm forgetting? Sometimes it's hard to tell, unless you look at the back of the box. Here is a picture of a box of the Highway 61 Hurst Olds, and my Motor State 66' GTO, tooling by Lane, and made for Motor State by Highway 61. Some of the early American Muscle 1:18 cars are pretty basic, but then again it was early 90s and not many companies made 1:18 diecast cars. Their later, post mid 2000 American Muscle line-up of cars are actually decently detailed considering they don't cost a fortune. I can't comment on Welly since I only have 3 of their 1986 1:18 scale Mustang GTs that are extremely rare and I paid $15 for them at KB Toys before they went out of business and that model is very well done for a lower quality type of model. I also have a 4th gen. Welly BMW 745i, it's a nice model for a lower end quality model IMO. I've seen some on eBay selling for $75+, and I have no idea why. It looks more like a $30 model to me.
Do you have a link to each model you want to buy?
Here's picture of my Highway 61 box (Black one), and the Motor State GTO box (White).