Axle back is referring to just the muffler. The rest of the piping is stock. So you're just buying the noise and some performance. The Cat-back refers from the catalyc converter to the exhaust tips. The Borla is better because it's a cat-back exhaust. It would have been better if it was a header exhaust, but that requires custom work.
Cat-Back:
Longer
Better performance-more air out and faster.
Better choice when you go to upgrade.
I don't think you want to tackle a turbo 8Cylinder, since it's rather complicated. Supercharging is the best. In that case, you'll want a custom exhaust.
I'm not 100% if the turbo exhaust from the SC3 will work on the SC4, but you can ask my fellow moderators (AWJ, and Lex_Luthor). they can answer that part.
My Choice, 92 SC400 w 4.9-li stroker, Tex headers, Dynamax mufflers, & 2.5" true -dual. Best bang for your buck.