I've never based on a which move is better by PP, but I would choose Roost over Synthesis because, as you said; the weather variability.
Think about it; When there's no weather, Synthesis heals half. Same goes for Roost. What makes Synthesis better is that it heals 2/3 health instead of half during harsh sunlight. However, this is extremely situtional, since weather teams are not that common, and the chance its that it will be a drought team? Mostly, Pokemon I've seen to be useful from experience who is always used to change the weather are Hippodown and Mega Charazard Y (and if you count Pelliper who recently got Drizzle). "Mega Charazard Y! That's a drought user." you might say, but to be honest I've seen plenty more Sandstorm and Rain users on Showdown, which will make you heal less than half.
Compare that with Roost, which its only downside is getting rid of your flying type or levitate ability. Decidueye has neither, so it wouldn't hurt it.
Hope I helped!
Edit: Checking the Moveset Question the ratio between Synthesis to Roost is 3 to 5, so Roost is only a majority by two, which I don't think is much of a difference.