Last Fall I looked at them all. There are basically two different kinds of head drives which are gear or shaft. Kuhn uses gear drive as well as many of the other European brands. It's a solid design and if you look at old cutters those heads are usually still pretty tight. They do take more hp though and maintenance issue discussed hear at length can also be an issue. I've also heard that mowing on a hill can be an issue as the cutter bar shares the oil reservoir though the dealer assured me this wasn't an issue. Still, if you get a leak you might loose them all.
The shaft design was originally designed by Lely. AGCO/Hesston/MF, New Holland, Lely, Vermeer and maybe others use this design. The AGCO/Hesston/MF head is made by a company I believe from Italy that I would consider a second tier manufacturer so stay away. New Holland makes their own, (now), and Vermeer gets theirs straight from the horses mouth, (Lely). The old white colored New Idea mowers also had Lely heads. Haymagnum mowers are made in USA with the Lely design but only have the 3-pt kind which was too small for me.
I went to a bunch of auction places and dealers and looked at their heads after a lot of use. The older New Holland, (Kuhn head), were fine but the newer ones all seemed loose. The white New Idea head was excellent even though the machine looked beat up and that was the Lely desgin. So, I narrowed it down to Kuhn and Vermeer as the Hay Magnum was too small for me. I ended up ordering a Vermeer TM850 which I'm hoping is here in the next week or two. I like the Kuhn but one day my dad and I drove 4+ hours to Vermeer and even though it was an off day for them they made room for us to get a tour. We even met with some upper management guys who really new their hay stuff. Getting service like that is unheard of. That, and the fact that I didn't like the Kuhn trailed mower design with the springs for head float, made Vermeer a winner for me.
IMO - there is difference between cutter heads. Kuhn I belive first designed the gear bar and Lely the shaft so I wanted to stick with them. Also, as mentioned I think the NH is better than the AGCO/Hesston/MF head but the Lely was clearly better than both of them.
I'm a big made in USA guy to so even though the cutterhead is foreign, (Lely), the Vermeer is made in USA. I believe the entire NH machine is made in USA which is a bonus for NH IMO.