To be honest, I'm most fond of a solo that has multiple attacks in a standard action and instead of being dazed/stunned it loses an attack. For example the Heroslayer Hydra loses 1 attack per daze/stun condition upon it, but otherwise isn't bothered by daze/stun. I like this a lot as it keeps daze/stun useful but it isn't about to be dominating the fight. Predictably enough said Hydra was a good challenge to an epic level party - when you can't lock it down and it has actions you end up with a great fight! Who would have guessed?
I'm also a fan of multiple initiative solos where a daze/stun removes an activation instead of normally affecting it. These solos are the best designed in 4E are coincidentally also produce the most fun battles.
This is where I think a solo rule that generally affects daze and stun conditions would be the best. It's action denial that truly screws solos and not general debuffs (though they don't help, in the end if you can't act at all you can't challenge anyone!). Making save ends effects better than "until end of next turn" is another entirely separate issue with its own problems. In reality save ends effects prove to be very good on normal monsters, but are worthless against solos due to their +5 saves. If you basically sabotage "until end of next turn effects" being better inherently on solos you make that balance much better anyway.
Because in the end the main advantage of "until end of next turn" is on solos. That's why you see PCs proliferate these, because the biggest and supposedly nastiest monsters are most trivially defeated with them. If solos are saving either way, then save ends becomes automatically better. If they're going to fail a save then save ends means it stays around longer, while they're going to have the same duration (or less) with an "until end of next turn" effect.
Against regular monsters the 50% chance of the effect continuing to last is good enough to keep save ends competitive. Change this dynamic with solos and you'll find PCs will be wanting to miss less with save ends effects on hits (and until next turn effects on a miss).