Swings and roundabouts I guess.
The diesel should be cheaper to run, better mpg, cheaper tax and quicker.
The petrol sounds better (still rather muted so induction/exhaust would be a good upgrade), smells better and potentially better in the corners. You'll also deffinately not be embarassed when someone asks you "what engines in that mate"
I don't own a brera but I do have a GT. I've had the 2.0JTS and now the current 170bhp diesel. To be honest, I could barely notice the extra weight on the front end round the twisties and since modding my diesel it goes really well and is much quicker than the JTS on all roads. On a motorway run it'll return around 50mpg and is cheaper to tax.
The downside of all this is that it stinks, sounds terrible and doesn't look right puffing out a load of black smoke. I've upped the power to a point that the clutch/DMF is now knackered and will cost about a grand to be replaced with uprated parts.
I'm not really sure wher I'm going with all this
but I guess what I'm trying to say is that the diesels can be quicker than the petrols but may have reliability issues with things like clutch/DMF etc.
I guess I'm back to the swings and roundabouts point. If you want the most power/speed you can, get the diesel but be prepared for some suprises. If you can sacrifice some speed and mpg's then go for the petrol variant.
In the end, it all comes down to personal preference and the things you need/want most out of the car.