My rule of thumb is 7 pto horsepower for each head on flat ground. It takes a bit to get one going, even my 100 pto tractor balks a bit on startup (blows a little smoke). Always a cab tractor. an open station is an invitation to get hit by tocks, wood or whatever that the discs throw forward.
I actually have 2 mowers, a sickle bar conditioner and a disc bine. I use the sickle on fields that I'm not sure of whats under the hay (like a new customer). I never run the disc machine on new fields.
Typically, I can mow twice as fast with a disc compared to a sickle and plugging in tall, damp forage isn't an issue, however, the sickle produces a better close cut on 2nd and 3rd cuts versus the disc.
If fuel is an issue, always run a sickle. Disc machines suck fuel, much like round balers versus small square bailers.
Finally, if in doubt about any poly twine left in a field (from missed bales, run a sickle. Poly twine is a quick death for the upper seals on a disc machine.