Everybody has an opinion as to what they prefer but its you who will need to make the final decision. As mentioned above, if feeding cost is a concern the 9mm is the way to go but for fun factor, the 45 is the choice. I looked at the XD myself but ultimately I chose the P345D in stainless and have not had a regret. After 500+rds it has not experienced a failure and the initial out of the box hard trigger is increasingly smoother with use, now to the point where it is very easy to operate.
Cheers
dB