Best to wear a bag over your head while driving either one.
These two cars don't appeal to the same type of buyer. One has got the cash to go premium while the other needs to watch the wallet. Both, however, offer decent enough interior space, and, well, that's about it. The only other thing they have in common is that they're both downright ugly. The BMW 5 Series GT and Fiat 500L fall into the category of "What the Hell Were They Thinking?!" when their respective automakers gave the production green light.
If you want a BMW 5 Series with extra space, then go for the wagon variant. Don't live in Europe? Fine, the X3 or X5 will work. Want a Fiat? No problem. The 500 is a fine small car and the Abarth variant is a hoot and a half. But stretching its wheelbase and turning it into a people mover was a stupid idea. The Nissan Juke or even the Kia Soul look better. So which in your opinion is the uglier one? The Bimmer or the Fiat?