I wanted to know whether I should use Blissey or Chansey on my OU team. I asked around and searched popular fourms and, although I decided to use Blissey, I wasn't completely satisfied with my answer and decided to make a more in-depth comparision myself.(Plus, I was looking for an excuse to Chansey as I prefer true-blue Gen Is over Gen IIs line extensions) I am providing my comparison below to help those qlso unsure of which to pick. By the way, I haven't played on PO since last Summer. So as far as popular choices are concerned I may be inaccurate as I'm sure the Excadrill and Thunderous ban has changed the metagame.

**Wish**: Blissey does pass bigger wishs than Chansey due to her higher total HP. However, the difference in total HP is only a mere 10 Hp (Blissey has 714 and Chansey has 704) meaning Blissey's wish heals only 5 more than Chansey's. Another thing to note is that Blissey misses a turn of Leftovers heal when she switches out to pass a Wish.

**Special Attacks**: Blissey can use them, Chansey can't. However, with only a 75 base Special Attack, it's not always advisable. (I remember reading somewhere that Blissey's Flamethrower 3HKOs a Ferrothorn with Leftovers)

**Trick/Knock-off**: Due to Chansey's dependence on Eviolite, she is more vulnerable to these two moves than Blissey. All the same, being tricked a Choice item will severely hurt Blissey.The difference between the two being that Chansey is left with lower defences than Blissey. Chansey's and Blissey's base defences are:

HP: Chansey - 250, Blissey - 255

Defence: Chansey - 5, Blissey - 10

Special Defence: Chansey - 105, Blissey - 135

As far as non-Choice tricks are concerned, there are only two OU viable sets: TrickOrb Reuniclus and Trick Iron Ball Metagross. The former doesn't need trick to counter Blisssey and Chansey while the latter isn't very common and often runs Hammer Arm.

Knock-off leaves a more noticable difference as the loss of Eviolite for Chansey is like a permenant -1 stage in both defence and special defence, whereas Blissey's loss of Leftovers is like taking Sandstorm damage. However, Knock-off is a rare more that tends to only show up on non-rain Tentacruel. The move itself is fairly useless in OU due to the rarity of Eviolite which is its most rewarding target.

**Bulk or Leftovers**: the key difference between the two is Blissey's Leftovers vs Chansey's extra bulk. To compare the two I calculated the damage Chansey takes from attacks and the damage Blissey takes after two turns of Leftovers heal. The first turn for the turn of the attack and the second for a Protect stall. The results showed that the gap btween the two is a curve that is in Blissey's favor with weaker attacks and grows more and more in Chansey's favor as the attack power increases. Here are some of my calculations for physical attacks against Blissey and Chansey.

There are a few things worth knowing before reading the calculations. I like to be pessimistic when I Theorymon so I only calculated for the maximum damage Blissey and Chansey would take. If you want to know the minimum damage just multiply the maximum by 0,85. This may provide, however, a result that is inaccuate by one HP as I rounded down all of the results to have exactly the amount of damage you would receive in a real battle. The percentages are calculated based on that damage with there being a cut-off after two decimal places.(I didn't round them I just stopped writing them)

I didn't do very many calculations for special attacks as I figured the difference would be insignificant due to the massive special bulk of Blissey and Chansey. All the same, here are a few.

**Sandstorm**: Many feel that the popularity of Sandstorm in OU makes Blissey the better choice. However, this is not entirely true. Sandstorm does benefit Blissey (at least, benefit in the comparison) by reducing the amount of damage Blissey and Chansey can take thus slightly altering the curve in Blissey's favor. However, the gaps remain the same as Chansey receives as much damage from Sandstorm as Blissey heals from Leftovers with the change being that Chansey loses more instead of Blissey healing more. This change does provide an advantage to Chansey. Blissey has the opportunity to ause its Leftovers in non-Sandstorm enviroments by spamming Protect. However, Sandstorm removes this advantage and gives Chansey the chance to reduce the gap between the two by offering the choice to not use Protect and thus spare Chansey a turn of Sandstorm damage. (which is the same as canceling one of Blissey's Leftovers heal)

**Pursuit**: An odd point for Chansey that I have yet to point out is being Pursuited while switching out. Blissey suffers in these situations as she doesn't get any Leftovers heal to deal with the damage due to her no longer being present. Chansey, on the other hand, maintains every bit of her Eviolite bulk and can even escape extra Sandstorm damage. However, Pursuit is a rare move that you will probably only see on CB Scizors or CB Tyranitars.

**Misc.**:Blissey does have a few extra bonuses not already discussed to be taken in consideration. As already mentioned, Blissey has a little more HP than Chansey. This means she has an extra 10 HP to help her with her first attack and also means that any recovery that leaves her with over 704 HP isn't lost until she has 714 HP. This small bit of extra bulk also lets her heal 5 HP more when she Wishs or Softboils. Lastly, Blissey can snag an extra Leftovers heal when their opponent chooses to not inflict damage but instead switch out, boost, Protect, etc. Whether or not these small bonuses help to close the gap between Chansey and Blissey is fairly situational. For example, the 5 HP bigger Wish gratification won't help much if it's 5 HP over 714, means that you can't calculate 15 extra HP in your comparison as the larger heal can't benefit you over 10 extra HP. The extra Leftovers heal also has to suffer from the fact that it is dependent upon the play-style of your opponent and isn't a factor when Sandstorm is present.

One last final note for those considering Chansey. You can choose to give her 248 HP EVs instead of the standard 252 to leave her with 703 maximum HP. This does have the disadvantage of decreasing her Wish and Softboil heals by 1 HP and leaving her short an HP when taking her first hit or healing back to full health, however there are a few benefits. 703 isnt perfectly divisible by 16 whereas 704 is. This means that with 703 maximum HP, Chansey only takes 43 damage from Sandstorm instead of 44 as damage is always rounded down. this also reduces entry hazard and status damage by 1 HP.