I believe alloy can be lighter, but the main advantage of fibreglass is it is ideal for racing conditions as it is dead simple to repair ... while alloy requires a skilled panel beater.
Fibreglass will even flex a fair bit before it finally fails ... been there and repaired the damage

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The biggest weight saving will be replacing the doors, they are real heavy on a 105 ... but also in the middle of the car so the weight is between the wheels.
Pete
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ps: Remember it's all just opinions

'71 1750 Series 2 GTV: http://www.alfabb.com/bb/forums/showthread.php?p=208078