If you are o.k. with the longer drying times that a sickle or Disc Mower will give you, then my vote would absolutely be for the Disc Machine-we sell LOTS of Windrowers and Mower Conditioners and have everything from Alfalfa to tall Grass here-95% of our Machines are Disc Machines-they will cost you more up front, but over the life of the Machine it will even out. I think the Kuhn design is probably the best for a 3 point unit, but if you do decide that you want faster drying times by all means get an Impeller Conditioner- JD, Hesston, Kuhn all have their variations, but we see dramatically improved dry times with the Impeller versus Roll Conditioner, as well as less leaf loss.
If I were going to choose between the three Mower Conditioners with Impeller Conditioners listed above I would buy the Deere, simply because of the Cutterbar protection offered. I have had a bunch of Customers hit Fence Posts rocks, whatever with, say the Hesstion (which has a Kuhn Cutterbar on it) and it would cost the Customer in excess of $2000.00 to repair it, as you have to pull the Cutterbar out of the unit, take off all of the Hats, and split the Cutterbar horizontally-the Deere has a Safety Hub that takes all of 5 minutes to replace and around $40-45.00-
As far as speed goes, I have had a 15 foot Disc Mower Conditioner on the back of a John Deere 7820 in road gear cutting thick Alfalfa(1st cutting)-if you can sit in the seat (rough ground) and have enough power, they will cut incredibly fast-