The Diamond suit is a new suit introduced during this current Easter event.
Here are the comparisons :
1. Main stat :
Each piece (both Diamond and Lion) adds 28 to 29 to one of the main stat (STR/DEF/AGI/DEX).
2. Armor :
Each piece (both Diamond and Lion) adds 280-300 to Armor.
3. Contest Resistance :
Each piece (both Diamond and Lion) adds 48 to 51 to one of the combat resistance skill (Slash resistance / Pierce resistance / Blunt resistance).
4. Skill :
Each piece (both Diamond and Lion) adds 8-9 to one skill, and that skill can be combat, contest or politics.
5. Secondary stat :
I chose this last, since this is the differentiator.
A piece from the Lion suit will add to TWO secondary stats, both stats will be for contest (DMG, AMR, EVD, Crit).
A piece from the Diamond suit will add to ONE seondary stat. That stat can be war (ATK/Tactic) or contest (AMR/EVD).
Whether this stat is war or contest will greatly affect how good this suit is. See summary below.
Not sure if other secondary stats are possible from a sample size of 5, so if you have a Diamond piece with different stat, please message me in w5 (id : Sisland), pm in in forum (id : sisland).
6. Suit bonus :
When completed, the Diamond suit gives a bonus of STR +29, DEF + 29, HP + 1100.
The Lion suit bonus is DMG + 87, AMR + 116, HP + 1100.
The stat equivalent for the bonus are identical (STR + 29 = DMG +87 for the purpose of contest), but the Diamond suit bonus applies to all 3 categories (war, contest, politics).
If you want a suit for contest, the Lion suit is clearly better as it has at least one stat in contest more than the Diamond for each piece.
If you want a suit for politics, the blue Knight suit will give 325 stats, while the Diamond suit will give 200.5 on average. So the blue Knight suit, with is clearly preferred, and if you add a purple Knight gaiter, the comparison favors the Knight suit more.
Now, this last comparison will need a table :
|Blue Knight with purple Knight gaiter||689.5|
|Diamond with no pieces military||563|
|Diamond with 1 piece military||591.5|
|Diamond with 2 piece military||620|
|Diamond with 3 piece military||648.5|
|Diamond with 4 piece military||677|
|Diamond with all pieces military||705.5|
For non-donating players, the Diamond suit can be better than the blue Knight suit in war. This will depend on the secondary stat on each piece. If the secondary stat for all the Diamond pieces are military, then it will be better than the blue Knight suit with a purple Knight gaiter.
Another advantage of the Diamond suit is that it is not BoE.
For me, that advantage is too little, and the chance of getting all pieces with military stat too slim. I will stop hunting for this suit. For those who wish to continue, best of luck.