I'd honestly choose Fire x Spotted over Thistle x Spotted.
Both were bad, but I could at least stomach SpottedFire.
I honestly like thistlexspotted better than firexspotted. Its not pretending to be healthy. It actually has some set up. It doesn't suffer from telling and not showing. The relationship forms over time. its not just Firestar thinking sorting herbs is hot and deciding that equals love. Spottedleaf and Thistleclaw actually talk to one another. It is confined to one book. From a technical standpoint thistlexspotted is way better executed than firexspotted is.
But Fire x Spotted isn't exactly "unhealthy" either. Although creepy, it was mostly all from Fire's POV, his crush on an older cat, who died not long after. Spottedleaf was probably aware of his feelings, but never returned them while she was alive because she has a duty as a medicine cat.
While Spotted and Thistle is different. Thistleclaw was using Spottedleaf, and interacting with him at a painfully young age, barely out of the nursery age.
Firepaw's real age when he joined is up in the air cause he was a kittypet, but it's safe to say he was still a young cat, and had to start as an apprentice. He wasn't as accustomed to the rules of the clans yet either, so he probably didn't realize having feelings for a medicine cat, let alone an older one, isn't exactly accept there.
While Spotted and Thistle, were both born in TC, both raised into the code, and Thistle, in the end, chose the Dark Forest over Spotted, he never truly felt anything for her it would seem. He just wanted to use her. Sure it doesn't hide how unhealthy it is, but SpottedFire wasn't intentionally doing that either, cause it wasn't unhealthy, just not socially accepted and against the rules. While Spotted and Thistle could have been mates just fine if they had kept things going and Spotted became a warrior instead.
From a reading stand point, I'd find a pairing like Fire and Spotted more comfortable, because we see even more of their interactions after Spottedleaf died. But with SpottedThistle it to this book exclusively and from Spottedleaf's POV only, compared to SpottedlFire, we never really had a real in story POV from her, only that one prompt of thoughts she had. If SpottedFire was shown from Spottedleaf's POV, it'd have just as much or more weight than ThistleSpotted, but it doesn't because it's from Fire's POV instead.
In SH also, not much time has passed either, it could even be less time between Fire joining the clan and meeting Spotted, compared to her interacting with Thistle as a kit and then them ending their relationship. It didn't really last long, and a lot of fans have pointed that out before. While in time for the main story, it's been a while at least, and Fire had visited her den a few times. They've interacted, while she was alive, but I've noticed most people tend to ignore this in favor of comparing the development to SandFire.
And lastly, Spotted and Thistle is really inconsistant, and the Erins seriously need to stop doing this. Spotted and Thistle had zero interaction, or even implications of past possible romance between them, not even tension. It's not noted in BP, not even in OotS when both were very active duringt he drama going on with StarClan and the Dark Forest. The Erins could have put a tiny implication at least, but they didn't. And if anything, they just used Spottedleaf's character to finally try and give a legitimate reason why Thistleclaw went to kitty cat hell, even if it's a flimsy one.
SpottedThistle may have been dealt with better, but that only because they had their own novella focus, and improved writing after 11 years.