Let's get to the issue of the question. The 4.5" and 5.25" XDm's indeed have different sights, and the slide cut-out of the 5.25" is an issue.
The 4.5" inch has 3-dot sights, a very common sighting system that is very effective and usable as combat sights. The 5.25" has a fiber optic front sight, which is very easy to see. The rear sight is merely a notched black blade, which is adjustable. As competition sights this system is wonderful for accuracy, but for combat (i.e. home defense) purposes not so much. The lack of dots on the rear sight makes it tougher to acquire in chaos and a fiber optic sights has a greater chance of breaking in a struggle. For home defense, I think the 3-dot sights of the 4.5" win.
The cut out in the slide is great for lighter loads common in competition, but it leaves a hole in your slide that stuff can get into. In a more chaotic environment of combat, foreign matter in your gun probably isn't good.
I doubt the extra .75" of barrel, which gives greater velocity and a longer sight radius, would make a big enough difference in combat to overcome to negatives the larger XDm presents for uses outside of competition.
.45acp, .40 S&W, 9mm, .38 Spl, .380 acp, 22lr
12 gauge, .223/5.56x45mm, 30-30 Winchester
2 Chron. 7:14 Christians must seek His face