Well, for starters, Space Marines are chosen as children, tortured by SCIENCE!, and then drafted into an eternity of being monastic murder machines whose sole purpose is to hold up the crumbling foundations of an omnicidal dystopia in the name of a rotting carcass that eats psykers like chiclets. They're emotionally stunted orphans who were brainwashed and weaponized before being unleashed on a galaxy where EVERYTHING is trying to kill them. They never even had a chance to be people before someone turned them into a gun instead.
Stormcast, on the other hand, are dead heroes, chosen for their valour and faith, resurrected and sent to free the Mortal Realms from the abominations currently running the show, on behalf of a benevolent God-King (Though benevolent is seriously up to debate). They're traumatized heroes who had lives, personalities and histories prior to being crammed into primary colored hulkbuster armor and filled full of lightning so that they could go save their descendants from the eldritch horrors of a nightmare dimension. They endure death after death, losing a bit more of their soul each time, in order to prevent anyone else from suffering the fate which befell them.
One group are so far removed from humanity as to be utterly alien. The other group are so human it causes them pain. One group feels little in the way of emotion, the other group feels emotion as strongly as they did before death. One group hates and fears the alien. The other group allies regularly with space-lizards, skeletors and green monster-men. One group is the personification of the grim future in which they live. The other is a thing born of hope.
The similarities are cosmetic: big guys in easily paintable armor sell better than little dudes with fiddly bits. But the context for those cosmetic similarities is quite different. Think of it this way...Space Marines are Batman and Stormcast are Captain America. Both are super-heroes, both wear costumes, both punch bad guys, both save people. But they ain't the same, are they?
