Hi Dave,
I have had both Harken Lazy Jacks and the Dutchman Mainsail flaking system. Dutchman is on our current boat.
While there are pros and cons to both systems, if I had to state a preference it would probably be for the more traditional lazy jacks. The Dutchman system works reasonably well, but in my experience it takes a fair bit of tweaking to make it work as advertised.
Another consideration is that the Dutchmen system requires that the mainsail be modified with a series of holes through which the monofilament flaking lines can pass. This is not a big deal if you are ordering a new mainsail anyway. If you are retrofitting an existing sail, you could maybe do it yourself if you're handy, however it is often best to have a sailmaker familiar with the system make the modifications. Proper placement and alignment of those holes is key to the system working.
Whereas a typical lazyjack system does not require any modifications to the sail.