Whenever we get a question like this, I always assume the person in question has never actually seen a scythe in action. I’ll link the video from Lindybeige. This is what a scythe looks like.
Of the two, the broom is the better weapon and you can use it as an improvised staff. Plus, you can capture dust bunnies and other grossness in the broom head and shove them into your enemy’s face. Never underestimate potential revulsion as a means of psychological warfare to create openings.
Other improvised weapons better than scythes: fireplace pokers, pitchforks, shovels, and sledgehammers. If you need a stand in for the scythe, industrial shovels and pitchforks are pretty decent. The shovel especially.
The problem with the scythe is that the size and shape of the head unbalance it. You have to hold it in a very specific way and that way makes it unsuitable as a weapon. Well, unless your character wants to be waving a curved blade at their enemy’s ankles. Any other vector of attack would be a challenge.
Broom all the way, especially if it’s a sturdy, wooden broom with a hefty shaft.