I don't reload the 44 magnum, but stuff like this affects more than one caliber. There are lots of things that can cause little variations in pressure while working up a load. Using a different bullet, using a different primer or slightly diffferent OAL. Thats why it is important to start at a reduced load and work up to max.
For OAL the XTP is going to have a cannelure, sometimes 2, use the one that makes the correct OAL for your revolver. OAL is often misunderstood as a hard and fast rule when in all reality it is a guideline. Once again this is why reduced load work-up while watching for pressure signs is important. Honestly you could go with the maximum OAL your gun will still chamber, but if it were me I would crimp on the cannelure.
New gun, same ol' shot.