I have both Supertargets; the mk3 with williams sight, and the Osprey with williams sight (the army version) and the normal version with anschutz sight so I can compare. Both ospreys have the target stock. All 3 are tuned for shortrange targetwork.
The Osprey has the better stock ofcourse. Imo, the Osprey has the better trigger too, once run-in. Its less heavy than the mk3s.
The Osprey is much more hold sensitive though and has more recoil. The mk3 shoot like a dream.
Allthough I must add that the second osprey is smoother to shoot than my first. Dunno why. Maybe other tunespring used. Second one does have slightly heavier trigger. It hasnt shot much.
Indeed, tap alignment is crucial but thats with all taploaders, mk3 included.
I use them all for belltarget (like) shooting. And they all can shoot full points if I do my part. Especially the Ospreys can print some impressive groups too at that range. Most of the time, I get better results with the Ospreys.
The Osprey uses more universal front sight inserts than mk3. The Ospreys are easier to cock.
So why would people buy them those days? Because there's little wrong with them at that range accuracy wise I think. You dó have a disadvantage in trigger compared to Walther LG55 of course and because of its taploader design you have less efficiency so more recoil but that doesnt say much about inherent accuracy, only about ease of use. Its not that difficult to be accurate at 'only '10m. Its not an everyday rifle either. For some thats enough to buy it. LG55s you see for sale a LOT, bit like the HW97 plague of today
Dont forget what the Osprey really was; a hunting rifle with a weaker spring and target stock and sights. Its nót a designed match rifle. And as such, it really does a very good job.
I own a LG55T too and I use it for the same belltarget shooting as the above. The Walther LG55 was a breakbarrel design with its own inherent downsides.
The LG55 shoots like a dream but has a rare size inserts. Its quite easy to shoot though.
They all have their own charm and ups and downs imo. I wont sell any of them. Not while Im still alive.
Regarding value: Id state them at about 300 ivgc. Those with williams sight or with standard stock are more rare.

Anyway, take good care of it and have fun with yours!
Still hoping to add a normal stocked Osprey ST to the collection one day