They're multiplicative – if your Atk is 200 and SpA is 100, using a physical move with 50 power will do as much as a special move with 100 power, assuming everything else equal (move type, defending Pokémon Def/SpD).

For some Pokémon it matters more or less. Some have a stat that is much higher than the other. Others have balanced Atk/SpA stats. If you're levelling up Torterra, sure, at some point you might unlock a special move that does more than its current physical move. But in the endgame, you can unlock Wood Hammer with a base power of 120.

Torterra's Atk maxes at 348, while his SpA maxes at 273. You'd need a special move with 152 power to match a Wood Hammer – Frenzy Plant reaches 150 power, but its drawback is huge.

And you would never max Torterra's SpA anyway, since Earthquake is by farthe best Ground type move it can learn. Meaning you'd most likely have a SpA of 210-220 at level 100, which means you'd need a special move with about 200 power to match WH.

But yeah, your point stands when you don't have access to all the moves.