swordtails and platies are from the same genus, so they can cross-breed.
main differances:
-platy has bigger color variety
-swordtails are born bigger and more developed
-only male swords actually have the "sword"
-swordtails are slightly bigger than platies
-female swords are bigger than male swords, but platies are about the same size each gender
-swordtails are more sensitive (in my experiences, anyway)
-swordtails are also slightly less active than platies (again, my experience)
-swordtails take slightly longer to birth than platies, but i think it's worth the wait
actually, most of this is just personal experience, but it should give you a good idea about what to work with.