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It is generally good practive to overate electrical devices. A 120A alternator running at 110A continuous load is (in theory) MORE likely to fail than a 150A rated device.
Also (within reason) the heat generated by the alternator due to losses will be about the same on a 120A unit and a 150A unit.
This possibly leaves just the physical size and air circulation space. Or the 120A casings are better ventilated than the 150A casings.
The factor that limits the current produced by the alternator is the alternator itself. If the battery voltage dips due to a surge of demand (e.g. starting the engine), then the 150A one will run at 150A until the voltage recovers. The 120A one will only supply 120A but for longer. The peak temperature of the 150A one will be higher, but for a shorter time. Generally low temp, long duration is less damaging then high temp, short duration.
The 150A one will generate more heat than the 120A one when each is running at 100%.
I can't think of a reason why a Croma should need a higher capacity than the Vectra. Perhaps Fiat added a safety margin to the capacity, without realising that Opel put in one with just enough capacity for good reason.
Perhaps the 140A version may be a safer compromise - I'd need to look at what differentiates the two versions of the Vectra.