BlackOps: I've owned both and nowadays I no longer have my Mark 23. Amazing weapon. But to meet the requirements of SOCOM, the gun was incredibly over-engineered.
There is a guy back on HKPro that has a bunch of cool photos showing a Mark 23 side by side with a USP45T from a variety of angles and views.
EVERY single part of the Mark 23 is made bigger, thicker, stronger, and beefier. Consequently, the gun itself is quite a bit larger and heavier than the USP45 Tactical.
You could literally run over the Mark 23, drop it from a 10 story building, set it on fire, fill it with mud, submerge it for a year, fire it over 30,000 times, and the thing would never notice. Throw a magazine in it, and it would be ready to go. Truly an engineering marvel.
So why did I sell mine? I traded mine for 3 other guns including a P7 and a Colt Python. I'm not a Navy Seal or Army Ranger and I'm not planning on spending any time in Afghanistan soon.
I was looking for a home protection gun and the USP45 Tact fit the bill quite nicely. The Tactical was also cheaper, more light/laser choices are available for its rail, AND it has the same capacity of the Mark 23.
Do I miss the Mark 23? Oh yea. I'm smart enough to know that someday I'll have another one - for no other reason than that.
Good luck and Happy shooting!