transmission or the clutch?

[N1ceDrgn]

After an interesting evening for New Years I was on my way home and I noticed the car driving a little strange. I pulled over and checked things out and there was a 'box of rocks' sound coming from the underside of the engine?! When the clutch is in it stops and when it's out it starts? Pick up/ power is starting to fade too! At the same time it is difficult to put into gear sometimes. Before I pull it apart tomorrow, I wanted to see if anyone else had any ideas? Would it be the clutch or would it be the transmission?
There is only one other thing I noticed, the resevoir for the clutch was empty as well? I checked things out and I didn't see anything wrong on any of the lines. The only thing I did see is that the clutch slave cylinder was damp. Other than that, there wasn't anything. I'd like to hear what anyone has to say.