I've done a good bit of investigating on the issue of Blissey v. Chancey. Overall, Pre-Evo Stone Chancey is better than Blissey. However, there are exceptions and there is a story behind this. For my examples I am assuming a Bold 252 HP / 252 Def / 4 SpD spread for both Pokemon.
Chancey takes around 5% less damage than Blissey from special attacks. This can vary slightly based on how hard Blissey or Chancey is being hit. However, Chancey does not get Leftovers recovery. This means that in most cases Blissey will end up with around 1%-2% more HP than Chancey at the end of a turn after taking a special hit.
The big difference between Blissey and Pre-Evo Chancey comes when you look at how they each take physical hits. On average, Blissey takes 10%-20% more damage than Chancey. Again, variation is based on how hard Blissey or Chancey is being hit. Leftovers recovery should also be considered because Blissey will be regaining 6.25% of its HP at the end of each turn.
Now, I will look at a few situations where these two Pokemon differ. In Sandstorm or Hail Blissey is better since its Leftovers will counteract damage taken from the respective weather effect. Trick users should also be considered. Chancey loses a lot more of its edge than Blissey if it falls victim to a Trick user. That extra bulk will be lost. However, the Trick user won't benefit from Leftovers recovery.
It can be difficult to decide which Pokemon, Blissey or Chancey, to use as your special wall. If you hate losing Leftovers recovery, especially for a weather team, pick Blissey. If you like being able to take physical hits better, pick Chancey. The difference in special defensive capabilities is so minimal that it's not really worth considering.
Chancey takes around 5% less damage than Blissey from special attacks. This can vary slightly based on how hard Blissey or Chancey is being hit. However, Chancey does not get Leftovers recovery. This means that in most cases Blissey will end up with around 1%-2% more HP than Chancey at the end of a turn after taking a special hit.
The big difference between Blissey and Pre-Evo Chancey comes when you look at how they each take physical hits. On average, Blissey takes 10%-20% more damage than Chancey. Again, variation is based on how hard Blissey or Chancey is being hit. Leftovers recovery should also be considered because Blissey will be regaining 6.25% of its HP at the end of each turn.
Now, I will look at a few situations where these two Pokemon differ. In Sandstorm or Hail Blissey is better since its Leftovers will counteract damage taken from the respective weather effect. Trick users should also be considered. Chancey loses a lot more of its edge than Blissey if it falls victim to a Trick user. That extra bulk will be lost. However, the Trick user won't benefit from Leftovers recovery.
It can be difficult to decide which Pokemon, Blissey or Chancey, to use as your special wall. If you hate losing Leftovers recovery, especially for a weather team, pick Blissey. If you like being able to take physical hits better, pick Chancey. The difference in special defensive capabilities is so minimal that it's not really worth considering.