I have a 16' and 18' sickle machines and a 15.5' disc. The sickles haven't been out of the shed in two years, can't see myself going back. Ever!
The disc uses more power and fuel. I feel if comparing litres/ton or hours/ton the disc would kick the sickles butt. I have no data to back that up though, just a feeling. It also takes a bit more daily maintenance, but this requires less time than changing a couple of sickle sections, and waaaaayyyyy less than one good plug up. Money well spent, wish I'd have done it years ago.
Cutting hay is a much nicer job when the only time you get out of the machine is to have a leak, and a quick visual inspection why stretching the legs!
Check out AlbertBucks comments on sickle mowers he has some excellent posts on the subject, contrary to my own opinion.