Try changing:
<?php if (!(strcmp($rs_modelli->getColumnVal("modello_prod"), (WA_getSavedFormValue("persist_data","prodotto_reg"))))) {echo "selected=\"selected\"";} ?>
to:
<?php if (!(strcmp($rs_modelli->getColumnVal("modello_prod",false), (WA_getSavedFormValue("persist_data","prodotto_reg"))))) {echo "selected=\"selected\"";} ?>