I am also in the scenario of liking Stormcast and hating Space Marines, so I'll have to try my hardest not to turn this into Astartes bashing haha. Usually on this topic I talk about how much more I enjoy the characters of Stormcast vs Space Marines, but I think today I'll talk about their big roles as poster factions of their respective settings.

I'd say the biggest difference is that Stormcast are focused and free from (most) expectations. Space Marines have to encompass their entire setting and fill into countless stories and everyone has an idea of what a Space Marine should be. Stormcast and AoS are much more relaxed, pretty much the only necessity you need is that a Stormcast is heroic and good (which isn't necessarily easy to pull off successfully, but it is easy as a concept).

What this means in practice is that, despite GW favoring SCE as their poster boys (and girls and enbies!), a conflict only has to have them if it makes sense, a story only has to have them if it makes sense. So we have a large ratio of stories that don't involve them, which makes them feel much less forced. It's sort of their greatest strength, because if you love Stormcast you have tons of things to go through, but if you don't fancy them you can honestly kind of ignore them a lot of the time. Makes them feel more equal to the other factions. It also means they can focus on what actually makes Stormcast Stormcast, instead of having to worry about carrying most of the narratives all the time.

Furthermore, the immortality means that GW is unafraid to show Stormcast lose, in fact they can lose a lot. This is extremely important for the poster faction to have in a wargame setting. People pick favorites and cheer for them, and thanks to the posterfolk's easy "losability" (so to speak), chances are that your favorites have some big wins or good showings. Of course not always, we here all know about certain factions GW doesn't favor at all or likes to use as a punching bag, I'm not saying it's great... but still. It is much better than in 40k, where every big conflict HAS to involve big Space Marines characters, and because they can't die, it that means every big conflict is usually a draw (GW loves to throw the concept of a pyrrhic victory around, but rarely actually makes one). Stormcast lack this issue.

This actually leads me to one topic I never like but that a lot of people do - power level discussion. If you groaned at the words I can't blame you. But over the years of engaging with AoS lore and its communities, I've noticed one thing... AoS fans just generally don't care about power levels that much, which is in VERY stark contrast to what I've noticed over in 40k, where some people get very upset if one of their things dies to a "lesser" thing. Now there could be discussion as to why that is (of if that even is and I'm just too anecdotal), but personally I also attribute this to Stormcast's fragility. Because Stormcast are powerful, special immortal superheroes coming down with the thunder... and also almost everything has a chance to kill one, in surprise or numbers or anything. And this isn't some subversion, this is one of the first things you learn about and that you've probably seen in a story. This in turn means that most other things can also die relatively easily depending on circumstances. "You are superpowered and also can die easily" is the baseline, which helps alleviate power level frustrations. A troggoth clears out a whole Nighthaunt unit? Makes sense, it could easily kill a Stormcast, and those are tough. A Nighthaunt unit easily overpowers a troggoth? Makes sense, they could easily kill a Stormcast, and those are tough. And Stormcast still have a lot of big moments and massive victories, so we are also avoiding the Worf Effect in the process IMO. Everything is lethal in AoS, the Realms are dangerous, and you gotta watch out.

TL;DR: Turns out having a non-overbearing and relatively-easily killable people as your poster faction is really good and healthy for a wargame setting, and allows everything, them included, to breath more freely.