I have a video of some of the living San Miguel Grandmasters on my website: Follow the menu to the videos page. As always between different FMA there are a lot more similarities than differences but San Miguel is definitely different from the others and distinct in a very visible way. In terms of range San Miguel emphasizes the long range and the short range and views the middle range as an area to "get through". Also San Miguel makes different use of the stick and dagger to train the empty hands.
Compared to Modern Arnis, the training methodology is very different in terms of what is taught when and how things flow from abesedario to seguidas and later into free flow palusot and palakaw. The stepping in San Miguel is based on a diamond pattern that is different than the box stepping in Serada as well. Some other things to set it apart are the use of the long stick (32"-36") versus the 24"-26" stick of Serada. But also its emphasis on the generation of power and the specific use of principles to impart the style give it a distinct flavor. It is different in several ways perhaps most of all in its minimalism and lack of ornamentation compared to the other styles you mentioned. Having studied both Serada and Modern Arnis, I can tell you that it is not just the same stuff. As it is Doce Pares it has 12 basic strikes but there's a good number of subsets, supersets, and variations. It is not the korto kurbada that has come to signify modern Doce Pares but rather the linear old style Doce Pares "Original". San Miguel is a style of Doce Pares Eskrima, the personal style of Doce Pares Grandmaster Momoy Canete. I'm a San Miguel guy myself out in your neck of the woods.