Disc are the only way to go in my opinion. Change a sickle blade and then change a disc blade, your mind will be made up. Faster cutting, maintenance is nearly the same, fuel usage is more on the disc but the quantity of cutting will equal it out.
I would definitly prefer the rotary discs for swathing over the sicklebar any day!
I could imagine that sicklebar mowers is/were the prefered choise in america because there was a sickle bar on the header platform for the combine aswell. Simelar equipment is easier and simpler to maintain and getting parts for. Also i have seen pictures of header conversions where the header was mounted on the back of a regular tractor and used for mowing. I could imagine that the conditions it has to work under always would be very dry. If not, well then the farmer could easier wait a few days before swathing, than a farmer in northern europe could.
However, in Denmark where i come from, there is no doubt in my mind that rotary discs is only way to go. I work at a contractor and usually we have a very narrow time window to mow a considerable amount of grass for silage purposes 4-5 times a year. The last few years has been a hassle because of unstable weather which is why we invested in this rig:
It's a Kuhn FC883 Butterfly rig at the back and a Kuhn FC313 front mower at the front with the MF8480 in the middle. Depending on the ground conditions and field sizes, i run this baby over the ground at 20-25 km/hour or about 15 mph. It has a working width of 8,83 meters which is guess is something close to 30 feet.
The Massey's GTA3 console displayed its peak performance to be 24,3 hectars per hour consuming 60 / liters of diesel an hour, which i have a very hard time beliving could ever be achived by a sickle bar mower.