For those who want to know about +P in the Hi Power, I'd suggest reading an article by the HP user's guru,
Stephen Camp. Since I got my first HP in the 1970's, I've used Super Vel, +P, and +P+ and had no problems. Like Stephen, I now use 18.5 lb recoil springs, but only since about 2006.
The Browning Hi Power and 9mm
The author of the American Rifleman article, Anthony Vanderlinden, has an awesome book titled FN Browning Pistols, and the HP sections contain much information that can't be found anywhere else. It's pretty much the go-to guide for information about the history of the Hi Power.
For a carry 9mm pistol, I'd choose the MK III, only because of the better sights and safety lever. The cast frame is a bit tougher than the earlier models, but not enough that it would affect my decision.
Here's a 1988 HP that was modified a bit by Austin Behlert;