I used a bunch of the ball pass-throughs on my Midget Mustang. The steel balls were hollowed out behind the ball and the aluminum version was solid. As I understand it, steel is preferred for firewall penetrations so I used the steel as much as possible.
The benefit to the aluminum version is that you can drill it out (within reason) to the size you need.
The picture of the steel version shown in ACS makes it look like the hollowed version will look different when installed in your firewall. In reality, they're showing the backside of the fixture.
The only difference in appearance is that the aluminum version is more silver in color without the gold tint of the steel version. Even here, the difference isn't quite as stark as the ACS photos might suggest. Here's a shot of the aluminum version.
This version only requires a single hole, but is all aluminum so it probably won't provide the same level of fire protection. it's definitely easiest to work with.
Here's a shot of a bunch of pass-throughs on my Midget Mustang. The lower 3 are steel and the top one is aluminum.
Financially, all these buggers are a little pricey. This was one of those shocker items that I purchased towards the end of my project after I thought i was through spending bigger chunks of money. I think I ended up with 8 pass-throughs.
All FWIW because I'm certainly no expert on much of anything!