The standard should definitely be HP Fire for the exact reason of Sawsbuck, as well as stuff like Bouffalant, Ferroseed, other Grasses (Amoonguss), Bugs (Accelgor, Scyther, Durant), and Rotom-C.  Not to mention you basically get a second STAB if you're using her on a sun team.
HP Rock literally only gives you coverage on Fire-types (Charizard, Entei, Magmortar, and Moltres).
		
		
	 
Hitting Sawsbuck, Amoongus, and Ferroseed super-effective is nice (Bouffalant isn't Grass-type) and certainly worthy, but I have to contest you on the last bit there.  Rock gives outstanding coverage with Grass, hitting not only Fire but also Bug and Flying for super effective damage.  The only type that resists both is Steel, and Durant is taking 91.8% - 108.2% from +1 HP Rock, while Magneton is only hitting you on your boosted Special Defense.  You can also OHKO Insomnia Honchkrow when it switches in to counter you.  HP Rock hits those Grass-types neutrally, so you can at least take them down if they're weakened, though without HP Fire they will definitely counter you at full health.  
Sorry if it seems like I'm correcting you when you were responding to my question to begin with, but I was really more curious about what is common on the ladder than what the benefits of one over the other are.  Ideally one would lead to the other but I know that's not always the case.  I don't really agree that HP Fire is always better than HP Rock, though.
As far as covering things that resist grass,
HP Fire hits: Steel, Grass, Bug
leaves you resisted by: Fire, Dragon
HP Rock hits: Bug, Flying, Fire
leaves you resisted by: Steel
HP Ice hits: Flying, Grass, Dragon
leaves you resisted by: Fire, Steel
I haven't been using my Sawsbuck team anymore so I haven't really been seeing what HP the Liligant I face have been running.  I'm still curious what people are running / seeing on the ladder.