Normally, the preferred location is the negative cable for most automotive applications. For the same reason you want to disconnect the negative cable before the positive and connect the positive before the negative; you don't want to accidentally hit the positive post with a frame part cause you will get a shock and sparks will fly.
In fact, those power disconnect switches that mount to a battery terminal are usually sized for the negative post (smaller) and won't fit the positive post (larger). But those are normally designed for under the hood of a vehicle, where in most cases, the battery positive post is very close to the frame of the vehicle. (On my Silverado, not sure you can remove the positive cable without a good possibility of hitting the frame, so disconnecting the negative first is a must.)
And if you disconnect the negative, there is NO current flow through devices, so even though there is a power potential at the devices, power cannot flow through a device without a complete circuit.
In regard to the power winch, it has it's own separate power and ground, which are normally connected separately to the battery and a disconnect switch is usually used to disconnect the trailer wiring.
But with a trailer where the battery is in a box, chances are you won't be messing around inside the box near the battery posts, so either would do.
On mine, the inline fuse is in the positive side and I just pop the fuse to disconnect the circuit to the trailer/tv/breakaway. So if installing a switch in the same location; don't forget to close the circuit to allow power to the breakaway switch.
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!