Not got Last Word so cannot comment yet (bring on Home time!)
On PvP though, my use of Thorn is mental. I can get stupid amount of kills if I run conservatively as 2 to 3 shots, plus the lingering effect kills my target whilst I take cover after shots.
The last word has 2 more bullet and it fires as fast as you can pull and very stable with the right perk. So you can nail 3-5 kill shots quickly, and reload faster than Thorn, especially with Chain of Woe.
Get Last Word but still work on the bounty, then decide which you like.
I heart my thorn but I want TLW just so I can Robocop whenever I kill!