Yep, I agree with this. Prime Surtur is more powerful than Hela, but Hela is definitely presented as being the bigger threat to the Universe at whole, which is exactly why Thor went to the lengths of unleashing Surtur in the first place, in this context he was the lesser of two evils.
The whole reason for Surtur's existence, his destiny, is to destroy Asgard and bring about Ragnarok. Therefore, he's insanely powerful and dangerous but his scope is more of a 'localised' threat than Hela's. Her plan was to firstly conquer Asgard (which she did with ease), then the Nine Realms and then move on to the rest of the Cosmos. The longer she remains on Asgard the more powerful she gets, we saw how powerful she became after only spending a
couple of days there, think of how powerful she could have eventually become. She would have become an Odin level powerhouse and easily could have moved from planet to planet, slaughtering and destroying whole civilisations.
If we're talking in terms of TV Tropes (which we should always do because it's awesome), Hela is the
'Big Bad', she's the cause of everything bad that's happening to the heroes and the catalyst for the main storyline. Surtur is a '
Greater Scope Villain' (kinda) and the Grandmaster is a
'Starter Villain' (kinda).