Raising porch floor to accommodate plumbing and keep first floor all one level
I generally prefer the same floor level too, but it sounds like this is separate from the plumbing. If either route would let you get the plumbing installed (to code), I don't see any reason not to.  Also keep in mind that you can raise the floor and run the plumbing straight down through drilled holes.  

As far as raising the floor goes, the main consideration there wouldn't be the plumbing but whether the ceiling is high enough after you raise your floor 6 inches or so. IBC Section 310.6.1 would require that there be at least 7' clearance to the lowest projection from the ceiling (i.e. the bottom of the light fixture). If space allows and you'd be happier with the result, I'd go for it.