There are advantages and disadvantages to each approach...a gas filled cartridge will definitely perform well, but if it leaks, you're hosed. A cartridge that uses discs/valving and fork oil is perhaps less prone to a total failure over time, but does require servicing at regular intervals (every 25K + or -) just because the oil will "wear" over time and become a bit contaminated from bushing wear (HD part).
Traxxion Dynamics AK-20 cartridges is the more expensive option that is not gas charged (they do produce a gas charged cartridge for racing applications). My last conversation with Traxxion indicated that when they have torn down the forks on bikes with their cartridges installed to check for wear and to change the fork oil, there has been no measureable wear on their components. They machine all components in house, so have total quality control. Most of this information is on Gold Wings which they have done thousands of, as opposed to only hundreds of Harleys.
From reports here, it seems that if you list Traxxion as a 10 out of 10 with regards to performance, the monotubes come in at 7-8, depending on who you talk to that has the knowledge to compare the two. I have no personal opinion on the Monotubes, as I have not ridden a bike with them installed...I'm just parroting what others have said. I went with Traxxion because they are a known entity for me, as I had them on two other bikes previously...plus, I know the people at Traxxion are top notch folks, have a great product, and are relatively local to me (within 200 miles). Having said that, I have never heard a bad report about Progressive's customer support.
IMO, there are really only three options on the table for a good front end...Traxxion, Progressive, and the new HD Touring Cartridges. Changing springs, fork oil weight, emulators, etc. will make improvements in some respects, but do not compare to a true cartridge system.