Which do you think, in your opinion is the harder boss?

Professor Putricide 25 HC or Sindragosa 25 HC. I consider them an equal level of difficulty with Sindragosa much harsher if you make a mistake (1 shots you with Icebombs and Blistering Cold) but has lesser mechanics than PP. But PP, you can afford some mistakes but is longer and has more mechanics to deal with.

On PP its good enough to have a raid leader tell people when to push phases and when to stop.
On Sindragosa everyone has more personal responsibility in case of Unchained magic, Blistering cold and Ice tombs.

So Sindragosa is harder, At least in Heroic.

Hardest boss in WOTLK with the gear provided from the raid itself -Mimiron hardmode-

I think that Disktrasa has pointed it out well. As neither of them might be hard for you or most of the RAID, there’s a much higher possibility for a wipe on Sindra as one guy can wipe 1/2 of the RAID, depending on stacking technique.

PP is usually easier because you have overgeared people to burst through it. But you can make Sindy easier by getting more RDPS and DKs (whom can anti magic shell through most of the mechanics.)