This page advises where you should plant the carrot seeds, and which Pokemon each results in.
Follow the walkthrough for Legendary Clue 1 to get to the point where you need to choose where to plant your carrot seeds.
Glastrier is an Ice-type Legendary Pokemon, and Spectrier is a Ghost-type Legendary Pokemon.
If you plant your Carrot Seeds in the frozen soil at Snowside Slope, you'll end up with an Iceroot Carrot - which will summon Glastrier.
If you plant your Carrot Seeds in the haunted soil at the Old Cemetery, you'll end up with a Shaderoot Carrot - which will summon Spectrier!
Spectrier is a Ghost-type Pokemon with high Special Attack and Speed, but low Defense.
Overall, Spectrier is the better option of the two - so long as you have some other Special moves you can teach it to take advantage of it's Special Attack and Speed in battle