Here we go...
The Mk III and the Practical share many basic features. I sort of consider the Mk III to be the "base gun" for this newest version of the Hi Power.
The majority of Mk III pistols have the internal firing pin safety. I am not aware of any Practicals that do not. The Practical and the Mk III use the same factory barrels and both have the magazine disconnect. Both have been manufactured in 9x19mm and .40 S&W but I have not seen the Mk III offered with adjustable sights. The Practical has been.
The Mk III usually comes with black, checkered nylon grips having thumb rests. The Practical's I've seen and shot came with checkered, rubber Pachmayr wrap-around grips and backstraps. Most of the Mk III pistols I've seen had the traditional FN ring hammer but not all; I've seen a couple with spur hammers like the Mk III. I have not yet seen a ring hammer on either a Mk III or the Standard from the factory. (The Standard is a Mk III sporting a bright blue finish and checkered walnut stocks. I have seen this model in both fixed and ajustable sight versions.)
Neither model has proven itself more accurate model nor can one be expected to always have a better trigger pull, at least not in my experiences with these guns. In other words, I do not see more handfitting, accurizing, etc. in one over the other. Both have the straight feed ramp and almost always handle about any JHP the shooter might use. Such could not always be said for the Pre-Mk II "classic" style Hi Power.
This shooter's Hi Power came with the ring hammer and everything but the slide, barrel and extractor have been hard chromed. It has the internal firing pin safety along with the magazine disconnect. Though not visible, this one sports the usual Pachmayr grips. Obviously, the hard chromed frame is the most visible difference between the Practical and the Mk III. This one is factory stock with no changes.The fixed sight version of the Practical has a more sloping non-serrated, ramp front sight than does the Mk III with its semi-post front. Fixed rear sights are the same on either.
Some runs of Mk III 9mm pistols have been with lanyard rings. I have not seen any Practicals with them.
Both the Mk III and the Practical have the "matte finish" on the slide. (This is a baked-on, black epoxy over parkerized steel.) The Practical has a hard chromed frame. The Mk III's frame has the previously described matte finish. I have seen some Practicals having the usual ambidextrous thumb safety levers hard chromed while they are not on other runs. Ditto, the triggers and slide releases.
I do not see either as having an advantage over the other in terms of reliability or mechanical accuracy. I think that the primary determinant for most shooters is which he finds more appealing be that due to looks or hammer style. The ring hammer is more visually appealing to many it seems, but it can also be more prone to bite a shooter's hand than the spur version, though both certainly can.
Checking around at both FN and Browning's sites, I do not see the Practical listed and have heard that this version is no longer being produced. I do not know if that's true or not. If it is true, I have no idea if it is permanent or not.
This 9mm Mk III has been lightly modified. Sights remain unaltered. Note the gun's semi-post front sight compared to the more sloping one on the Practical. I bobbed and recontoured the spur hammer on this one and had the gun blued and also removed the right-side thumb safety lever as they get in my way. It is wearing Spegel checkered delrin grips. Though slightly altered, one can see the differences between this lightly-modified Mk III and the Practical.
Speaking only for myself, my preference remains the Mk III. The reason is that the ring hammer definitely bites me more. I prefer the semi-post front sight on this version of the gun and can find it for less money. Other folks may feel just the opposite.