Never riden the P5 but the P5x is amazing, it is rides like a tank, which is how a Tri bike IMO should ride. my SC was slightly more twitchy, but still handles very well. I hear the P5d handles very nicely too, more road bike feeling.
TBH I am not too picky as I love to ride indoors a lot, but I also do a lot of long rides outdoors. The beam design is very nice for compliance and comfort. I also own a very nice road bike that I plan to do a lot with this year. I have yet to try the P5d or P series myself but I think they would handle great and im sure I could rock top bike splits with either as long as I get the same fit.