According to Smogen, Grassy Seed was more reccomanded:
Rillaboom is a mandatory teammate for Hawlucha, reliably setting up Grassy Terrain and bringing Hawlucha in safely via U-turn, significantly increasing its setup capabilities. In return, Rillaboom appreciates Hawlucha's ability to weaken its checks like Mandibuzz and Corviknight.
So, as you see, smogen was more reccomand Grassy Seed due to Rillaboom, and you also has a Rillaboom in your team as you mentioned. Also, I bet you are inspired by the set of MeloettaMelody to use Power Herb, but the Problem is that Power Herb can’t help if opponent outspeed and OHKO by other Pokemon.
Hope I helps!