Stats on both are random, so in any particular game one can be better than the other (I used the dragon armour). They are potentially a couple of the best breastplates in the game. Some people prefer the upgraded silver armour, since it is invisible on the character.
FYI, below is what I got for this armour;
Breastplate Of The Dragonheart
Durability 12
Charm Quality 3
Armour 440
2 Defense
100 Vitality
60 Magic
15 Resistance to Fire
40 Resistance to Lightning
15 Resistance to Spirit
4 Hearing
Survivors Instinct +1
Breastplate Of The Dragon - before the upgrade
Durability 30
Charm Quality 1
Armour 390
6 Strength
6 Intelligence
60 Vitality
100 Magic
15 Resistance to Lightning
15 Resistance to Poison
2 Hearing
Heaven's gift +1
Silver Breastplate - before the easter egg upgrade
Durability 23
Charm Quality 2
Armour 410
10 Constitution
15 Resistance to Poison
Polymorph +1
life Leech +1
Armour Of Larian - after the easter egg upgrade
Durability 19
Armour 410
4 Agility
10 Defense
40 Vitality
20 Magic
10 Resistance to Fire
40 Resistance to Lightning
20 Resistance to Poison
3 Hearing
Lightning +1