One of the most important things is to try the different lasts to see which of them suits you more if we are talking RTW.
John Lobb Prestige and G&G RTW for me are in line in terms of quality. My only concern with G&G is that the sole wears out pretty quickly but they are so great that I don't care.
G&G lasts and styles are more modern and John Lobb more classic. I prefer the latter for a wedding for example and G&G for everyday use.
Of course you have some G&G that are conservative:
And some modern Lobbs:
So both of them are cool, so you should choose taking into account the model you like, not the brand.