You still want to declare their season better than ours?
The Hawks are 1.5 games up with 7 games remaining, and they're 9-1 over the last 10. They have the 2nd best defense in the league. Their point differential is 4.0 compared to 3.4 for the Celts.
The Hawks also have more experience and a better track record of success in the playoffs (i.e. they've actually won playoff games with this group).
So yeah, I'm pretty confident that at the end of the day, the Hawks will have had an objectively "better" season than the Celts.
It's possible it goes the other way, but it seems to me that it's more likely for the Hawks. The point being, that it's not a simple conclusion to reach that Horford would look at Boston as such a vastly superior situation to the one he's already in.
If the Hawks offer 5 years at the max, I wouldn't be surprised to see Horford take that offer before seriously considering anybody else. If they balk at offering him quite that much, given his age and potential for decline, the Celts will have a pretty good shot at signing him, but I stand by my conviction that it'll require offering him 4 years at the max to make it happen, since plenty of other teams will be chasing him as well.