I assume that this is just pasture and you're cutting for maintenance sake and not for baling, etc....?
If so, the shredder (brush hog) is the way to go. More versatile and resilient. The sickle mower is really more a cropping tool simply cutting and letting lie the material whereas the shredder will cut/pulverize/spread the material cut. I have both and the sickle always is at risk of clogging and often times will simply ride-over smaller stem grass/weeds. The shredder, by design, does not clog and rarely rides above anything. You'll get just as much done with a 5'-6' shredder as you will a 7' sickle in a given amount of time because you can move faster.
Also, try pulverizing the odd small stump or fallen limbs hiding in the weeds with a sickle mower........:{