It's the speed of the rotary knives. Rotary swathers by design cut very fast. So the ground speed has to be higher or you're going to be 're-cutting' the crop before it's run thru the conditioner. The knives are much cheaper to sharpen/replace than a sickle bar, thou with a good 3/8" air wrench it doesn't take THAT long to switch sections. Just alot more money. You might be able to 'cheat' a bit and run longer with a duller sickle maching. Rotary swathers have to have sharp knives or they do a krapy job.  

The ground speed isn't the power hog, it's running the all the hydraulics involved with the head that eats the power.  Having good smooth hay fields are a real plus with any swather but rotary's work better on smoother ground so you can go faster. 

The real test is drying time. In this area,, there is a very minute difference in crop drying time. Really doesn't make any difference with the type of conditioner either, there's only a 1-2 day time difference window.

As far as "plugging up" a sickle bar swather all the time, if guys are constantly plugging up, they don't have their machines set up correctly. I run a sickle bar swather and haven't plugged up in 5-6 years. Or,, they are trying to go too fast for the material being run thru the conditioner.  

If you cut alot of hay 700+ acres per cutting, and want to cut fast,(9-13 mph) rotary's will do that. With,, good flat ground and do a good job.  If you've got "lumpy" fields,, and/or less acres, a sicke bar will do a good job too.  In this area, Rotary machines have allowed guys to trade off 2 sickle bar machines for 1 rotary machine and still be able to whack down 100+ acres per day.

Rotary machines just allow you to go allot faster if ground conditions allow it.

Sickle bar machines are more forgiving of lumpy ground variable crop conditions. When a crop is really down, (flatter than a pancake) sickle bar machines can pull the crop up a bit better 'going with the grain' thou it's slower going. I've got a couple of fields of 5-6 ton timothy that's barely 9 inches high. TOO much high wind sucks.... (NOT looking forward to any brownleaf possibliitys...)

Watched the neighbor running thru 3 ton alfalfa the other day at 10-12 mph. Cut fast for sure.. then stopped 1/2 way thru the field and hasn't cut any more for 3 days. So,, not so sure that 'high speed cutting is great' if can't put down an entire 40 acres in one afternoon..??  Yes,, they always run a rock roller across all their hay ground,, smooth enough to land an airplane.

IF you're doing custom work and thinking 'alot faster with a rotary' (which is true) only issue is not all farmers have nice smooth ground where the speed can be used. Not fun running a rotary when 1/2 the time your rear end isn't on the seat.

Either will do the job.  Conditioners are as much/more important than the cutter bar for drying time.

I would highly suggest you try out both types of swathers before buying.