So you have neither? I would say use a scythe as the job will go quicker, but a scythe is a larger investment.
I bought mine from scythesupply.com. Awesome selection, they size the snath to your personal measurements, and they quickly answered any questions I have. You're looking at $200 to get a package though.
A sickle will do the job with a bit more labor, and be cheaper. You can also buy sickles on their site.
What I would do, since you have neither, is quite a cheaper option. Do you have any
Tractor Supply Company stores around you? They
sell a "corn knife" which would be perfect for what you're looking for (chop and drop) for $6. It's basically a sickle-like machete. Comes respectably sharp out the package and has served me well for similar tasks.
50x4 beds will take longer to chop than you're probably expecting though. If you have any other use for the scythe I would almost suggest getting it. It'll definitely speed up the job.