The difference between the 2 on performance is hardly noticeable. Astra quoted to do 0-60 in under 6.5 secs, alfa in 6.3. Astra quoted to do top speed of around 150, Alfa will do 153.
Saying that, I've heard the price of the Astra is going to be around £20,000 mark, whereas the alfa costs £22,750. Thats an expensive .2 of a second to 60 and 3mph extra top end @ £2750.
Also the Alfa is powered by an NA V6 engine which is going to be far more expensive to extract extra power from than the Z20LET is.