Sequels vs. Prequels

Since there are more and more movie adaptations of comic books coming out there's a slew of storylines and characters that may very well get a chance to shine. But what's the best way to really expose characters to a potential new fanbase? Prequels like X-Men Origins: Wolverine flopped, whereas sequels like Dark Night and Iron Man 2 made a killing. 
Are basic origin stories the best way to start? Batman Begins = Good; Spider-Man = Shoddy Attempt 
Or are sequels the best way to introduce new ideas and characters? Black Widow made a debut, as did Silver Surfer, Joker, Two Face, etc.  
 
So which is better?

Going back or foward only have sense if the character have a place to go.  
The onyl reason to make prequels is like Star Wars to explain, where a character came and why he become what he is. 
But Sequels teach you, here he go and what he become. 
Then Sequels are better if you wish to show what the character is becoming and prequels to know where it was coming.

X-Men Origins: Wolverine is not a flop. It did not earn the likes of Dark Knight and Iron Man 2 but it earned. 
I don't know what is your basis of saying its a flop? Storyline? Characterization? or Just its plain gross/net earning? 
 
As to the thread: I find prequels intriguing but sequels give more time to flesh out the story and characters. I have yet to see another prequel that can surpass Starwars.

If you start with an origin story, theres nowhere else to go.
 
X-Men has a lot of possible back stories, because mutants don't have origins in the same way as typical Super Heroes. But Spider-Man? Nothing else before he's bitten that is worth talking about.
 
I just wish they would stop focusing on origins. In most cases we don't need them, and we certainly don't need them in reboots. Batman Begins is the main exception, since the reboot was almost entirely about how they dealt with his origin story. But this new Spider-Man? I really hope there isn't an origin story. It was one of the best decisions the newer Hulk made.

sequels!