The best way to judge what classes should have access to a spell is to compare it to the kind of spells those classes already have.
Wizard and Sorcerer compulsions tend to be forceful in method and drastic in effect: The examples you gave describe subverting the victim's normal mental process by forcibly inserting a foreign thought, and blatantly stealing control of the victim's mind.
Clerical spells, on the other hand, tend to use more straightforward methods of achieving their ends, and often express the role of clerics as spiritual leaders: Thus, the spells that allow them to speak with authority and be obeyed.
As for why the power of Power X isn't compelling... Well, if you look at the ecology of gods as described in, for example, the Planescape setting, belief is the food of the gods - without it, they lose strength and eventually starve to death, or at least enter a state that is for most practical purposes equivalent to death. A follower who only vaguely follows their patron deity's teachings is still more faithful, and therefore more valuable, than an unbeliever magically compelled to serve. Of course, individual gods might have a thing for magical compulsion, but that's not enough for something to get included in the standard priesthood package.
Oh, and like mxyzplk suggests, there's more game design-y reasons for it, too: if Clerics were good at all forms of magic, they'd be far superior to wizards, even enchanters - and playing an enchanter would be much less interesting.
My guess is that merely telling truth might work, as the suggestion could nudge the NPC into Bond Villain mode:
"these people are no threat, so I will reveal to them my great plan!"
"I can totally trust these people with this secret that I have been harbouring all these years!"
It does not ensure that the NPC will actually tell the truth, now. If, say, an innocent was Suggested that he reveal his plans when he actually has none, he might spin some up just to please the suggestion.
Your given wording, though, is not a good suggestion. A better one might be something like "You want to show us how dumb we are and reveal your real plans." Or "You absolutely trust me and will tell me everything you know." Your DM may actually like it, as it gives him a chance to roleplay a monologue.
Best Answer
This is ultimately subjective and up to DM ruling, but if you want a quick way to judge it, imagine that "Suggestion" is the name of a bottle of strong spirits, you just drank the whole thing, and now it's telling you that doing X would be totally awesome.
Ten years from now, will you be in the bar with your fellows, saying "Haha, remember that time when I drank a bottle of Suggestion and then X!"? That means it was totally reasonable, so X goes.
On the other hand, ten years from now, will you be waking up crying in the middle of the night, shouting "Why, why did I do X!"? That means it's not reasonable, so X doesn't go.
This would make giving away The One Ring very unreasonable, but giving away the magic ring you just "liberated" from some "evil creature" totally reasonable. It mostly makes it so the reasonable-ness of any act is judged by how much it's going to ruin your life and haunt you, not by any amount of monetary value or whatever, which seems to be the idea behind the spell.
For example, you might get a noble to give you his purse of gold, but not get a farmer to give you his only cow. You could, however, convince the farmer to dance naked through the streets, but you couldn't convince the uptight noble to do the same.