I bought the Giulia a year ago thinking I would never get a 5BW allocation that I had been trying to get since the prior August. As I was signing the paper work, the Cadillac dealer called me with an allocation and to finalize my order. So clearly the quadrifoglio is a good luck charm! I drove it daily from April until July then it became my wife's 3 season daily as I ended up with the blackwing.
The Giulia is more comparable to the 4BW, size, weight, HP. So comparison to the 5BW would be similar to comparing the 4 vs the 5. The Giulia is going to be the "better" car for track days or high performance driving. I'm not knocking the 5BW. It's just bigger and heavier. The Giulia always puts a smile on my face when I drive it or even just look at it. So does the blackwing, so that's a tie.
Now, for my use, the 5 is the better sedan. We use both sedans to transport our 16 month old and in August will be transporting a newborn as well. The 5BW has much more room. I prefer having a manual and gobs of torque/power. The blackwing also flys under the radar better than the Giulia. I'm not sure if it's the color or the color/car combo but the Giulia gets a lot of attention which is nice, but we prefer to blend in more.
Cold start winner: Blackwing
Ridiculous power winner: Blackwing
Sexier: The blackwing is pretty but the Giulia is definitely sexier.
Driving: I'd happily take the keys to either car and I feel very fortunate to own both. If you have never driven a Quadrifoglio I highly suggest you do.
Both cars are fantastic machines. If I had to let one go I'd miss either one, but would still be happy to have whatever one survived.
Hope this helps.