where you get married does not really  matter but i would probably advise that either she file a fiance visa on your beahlf and then you enter the US on that visa and get married or you get married and then she petitions for you as her spouse.  even if you use your italian citizenship to enter the US, you can only do that for 3 months on the visa waiver and that is not enough time to get a green card so you would still need to start the process of applying for adjustment of status but then it may be seen as preexisting intent to immigrate while entering the us on a visitor/nonimmigrant basis.