Hi lads, wanted to get my diff painted so its ready for when I put the rear end back together but I'm unsure which to use...

The one that was in the car when I stripped it down, or a good used one I recently brought /

Worried the one in it had a bit of swarf on the plug when I changed the oil but worked fine, or the stick the 'new' one in which I have no idea about....

Is there any checks I can do while they're both out on the floor?

Cheers, pete