While Thorn may be more accurate at longer ranges, TLW PvP wise has a much faster TTK (Time To Kill) at short to mid range. When combined with a Sniper, and a good aim, a Guardian can go on multiple kill streaks if used correctly.
Now, PvE wise, Thorn is the better choice.
There are multiple reasons why thorn is better:
A. Thorn is more accurate at longer ranges in PVP and PVE.
B. Thorn's Exotic Perk, mark of the devourer, is more useful in getting postmortems in PvP and killing bosses quickly in PvE, whereas the namesake exotic perk for TLW only increases accuracy and precision damage whilst hipfiring (it is habit that I aim downsights, so I almost never use this perk)
C. Thorn is actually decent for PvE, unlike TLW, where thorn can keep enemies like Captains and Minatours from regenerating thrir sheilds while dealing massive damage from mark of the devourer (which can stack to do more, up to 5 times) while TLW does not deal NEARLY as much damage to enemies as thorn, and not at as far a ranbe with as much accuracy.
D. Finally, Thorn just performed better than TLW. I could rack up close to 30 kills a match with Thorn and kill bosses fairly easily, whereas TLW has only gotten me at most an 8 kill match in PvP and can only do so much to bullet sponge PvE brutes.
Just face it: Thorn is just better than The Last Word in almost every aspect.