What are the pros and cons of using 2 layers of 210gsm against 1 layer of 416gsm, apart from price and ease of forming :-) ?

Theoretically speaking, two layers of the 210g would result in a laminate with less crimp, making it stronger. Realistically, this is such a marginal difference that it is really irrelevant for almost all applications, especially considering that if you were concerned about such things then you would be using unidirectionals not wovens. You also aren't at structural application thicknesses with such a layup, so the application sounds cosmetic with no real concern for such tight strength tolerances.

Aside from that, the only other benefit is the ability to rotate the fibre orientation to achieve strength in multiple directions. This is probably more important than the above, but it depends on what you're making and where you need strength. 

For the most part, the time savings from only having to layup the single 450g ply far outweigh any other considerations, which is why it is a popular layup.