This is how they differ. When you create a darkroom spawner, it relies on the standard mob spawning range of (i think) 120-ish blocks. This means that, when you are within 120 blocks of the farm, it will activate, and entities will path and there you go.

Here is how spawner blocks differ. While the mobs, once spawned, will have their normal pathing, what's changing is the activation radius. A mere 16 blocks (or whatever the activation range is on the spawners currently, but I know its a small radius). Which means farms based on spawners will not activate unless players are 16 blocks from the spawner, while a darkroom farm will activate within 120 blocks.

For players who farm a lot of mobs and have large darkroom spawners, switching to spawner blocks would eliminate a lot of pathing that's going on when they're just "in the area".

Here's what I think:

Spawners should be somewhat less frequent (only a little), but they can be moved.
Either make it really expensive to do, or let there be a limit of, say, 5 spawned blocks per chunk (I vote first one)

Dark room spawners should be de-opped somehow, maybe make witches spawn in with lightning strikes? It would temporarily light up the spawn room (the fire), reducing rates for a short amount of time. The lightning should not damage players, as that is not the point.

These were just some small ideas I had to reduce the need for laggy dark room spawners.

Honestly, it wouldn't take terribly much to make dark room mob spawners be able to be turned on and off, and we can make that a requirement. I have a dark room mob spawner, I -could- add in the circuit, I just haven't yet because I'm busy with other projects :p

I could add a standard one in for you tomorrow, if you would like