The process of her becoming a US citizen required her to renounce her Philippine citizenship. Once a US citizen it is a simple step to reaffirm her Philippine citizenship without affecting her US citizenship. Thus dual citizen vs one or the other.
Going the SRRV route, she is not a Philippine citizen and does not have all the rights of a citizen (voting, unhindered land ownership...). However, depending upon what your plans and desires are the limitations may not be an issue. On the positive side, if you do not qualify for the extended courtesy SRRV on your own (former military...), you do qualify as her spouse (as stated above) and do not need to add to her reduced $1,500 investment requirement.