The shield can be a lifesaver if you find an opponent getting the better of you. Or if you're swarmed by enemies, mildly disoriented, and wish to just get everyone off you until you can regain your bearings. I use it in those instances. Emergencies. I don't use it for the purpose of avoiding combat wherever possible. It's also an amusing time-killer if you just feel like picking a fight with city guards, especially coupled with the Shield Charge perk.
The crown I can only see myself selecting if I were playing a non-warrior character, and then only for the look of it. I would hardly agree with you that the crown is "probably most powerful" item in the game. It allows you to stack two Standing Stones abilities. That might actually be impressive if the Standing Stones abilities didn't svck for the most part. Three of them are "learn faster" abilities which will eventually outlive their usefulness. Four of them are "once per day" abilities which makes them only marginally useful and one of those (The Tower) is the lamentable "Save yourself some lockpicks once per day!" ability. Yes, save yourself on some lockpicks. Because we all know how rare and expensive these curious little items are. I honestly don't think I could conceive of a more crap ability on my own without putting some serious effort into it.
That leaves only a handful of Standing Stones abilities that are actually worth having for the duration of the game, but personally I think the average player is better off just double-enchanting their own headgear (if you're leaning towards the crown, then chances are you're a mage anyway, in which case you're likely to have (or planning to get) the "Extra Effect" enchanting perk).
I always choose the Crown.
I'll use your Warrior as an example...
The Warrior stone makes Warrior-centric skills go up 20% faster, which is helpful because an A-typical Warrior only really uses those Skills anyway. So this Stone is pretty useful.
The Steed Stone makes your armor weigh nothing, armor won't affect your movement speed, and you get an extra 100 to your carry weight... very, very useful.
The Lady Stone, perfect for a Warrior as you regain Health and Stamina 25% faster. Perfect for recovering from a grueling fight.
By mixing and matching these three Stones, your Warrior can have some very serious buffs before going out on his/her next excursion. This is the incalculable value of the Crown. The fact that most Stones don't help a Warrior a bit isn't the issue as the same applies for both Rogue-ish and Mage character builds. But it's the reality that instead of having to try to predict which Stone will help you more when you don't know what's facing you (when without the Crown) is mitigated by the fact that you can go into the unknown with the effect of two Stones just waiting to help you slaughter whatever is next.
I definitely go with the Crown for all of my characters.