A pin punctured my tyre. Should I only replace the inner tube, or patch the outer, rubber tyre?
The size of the puncture really dictates whether a new tire is in order... that being said, if you patch your tube and inflate the tire (the one with the puncture) you want to make sure the tube is not exposed through the puncture hole.  

If you need to ride and the tyre is questionable... an old trick is to put a dollar bill or power bar wrapper on the inside of the tire over the puncture hole... then install the tube and inflate.  The pressure from the inflated tube will be distributed across the wrapper resulting in an extra barrier keeping the tube from protruding.  

Good Luck.