There are lots of characters who fill the same niche.  Who would you choose between the following characters: 
  
Battle of the Explosive Babes - Dazzler or Jubilee or Boom Boom?
Fans and creators alike have argued that Jubilee was X-Men writer Chris Claremont's tabula rasa version of Dazzler, whose character was already largely defined by her groundbreaking solo series while Jubilee and Boom Boom have often bonded by their uncanny similarities in both power and personality.
  
Colorful Transporters - Nightcrawler or Biink?
Not much to differentiate here but general ability (I know they are used in different ways) and general appearance, but the fact that two of the X-Men's discolored mutants can teleport does raise a few redundancy flags. 
 
Brooding Dudes - Chamber or Rictor?
While their abilities are largely different, both former mutants can be characterized by their ability to brood over various tragic events in their lives.  While their circumstances are different, these dark-haired heroes still fill the same personality role and their similarities may make it impossible for them to ever serve on the same team. 
 
Bulky Guidos - Strong Guy or Rockslide  
These two characters have different visuals, but similar tank status in their respective affiliations.  Additionally, they both are the comic relief/dumb jock of their inner circles and both even are often paired with their own "straight" man (Madrox and Anole, respectively).
 
Pretentious and Elf-Like - Northstar or Namor  
While one is half-fairy and the other half-fish, these two often-waxed, high browed individuals are infamous for their snark on-and-off the battlefield.  Both share the ability to fly (among their other abilities) and have had trouble holding on to a steady gig in the past decade.  

Teenage Mutant Ninja Geniuses  - Prodigy or Cypher 
Since Cypher's return, he's taken over the role of young genius working on Danger and other "smart guy" projects Doctor Nemesis has no time for.  Both are former New Mutants of two different generations and have a history of being creepy, supportive, and boring in their comic book careers.  

Arrogant Playboys  - Sunspot or Hellion 
Both are from different junior X-Men teams and both have the ability to snag the ladies.  While Hellion's been cut-off since his self-titled mini-series in 2005, both men have been raised with a silver spoon in their mouths, spending their parents money on their mutant friends.  

Masters of Evil - Mastermind or Lady Mastermind 
This comparison is only natural and while these two ladies have shown to be able to co-exist and be interesting due to their scathing rivalry, its only a matter of time before one sister comes out on top.  

Unphased Youth - Cipher or Loa 
These two women share the ability to phase through solid objects, although one specializes in covert operations while the other can be a bit more destructive.  However, their general ability is duplicated in an already large generation of students and its only a matter of time when another crossover rolls around and another New X-Man has to turn in their shirt.  

Firey New Mutant  - Firestar or Magma 
Again, two lady mutants from the same generation share similar powersets.  However, one has been occupying her time in the larger Marvel universe while the other occupies her time...I'm not sure what Magma does with her time, actually.  

Maiden of Metal  - Husk or Mercury 
Most people would have trouble differentiating the two if it weren't for their brightly colored wigs on top of their metal bodies.  Someone has to go.  

Giant-Sized X-Men - Colossus or Warpath 
Warpath is more of a legacy to his brother, the original Thunderbird who debuted alongside Colossus in the 1970's, but nonetheless the two are large tanks in the X-Men arsenal.    Warpath's use had been growing steadily since he joined the X-Men during Brubaker's run, but he has drifted off the radar since leaving X-Force.  Can the younger spirit outshine the iconic if not stagnant Russian tank?

Quickly Catty - Quicksilver or Northstar or Velocidad (Gabriel Cohuelo) 
C'mon...the similarities are just too much.  Fast, snarky, and the former two even have similar eye-brows. 
 
Quicksilver has so much more relevant history to the X-Mythos but he is also oft-relegated to the Avengers or the removed X-Team X-Factor.  Northstar is an iconic gay figure in the X-Men.  Gabriel is part of an important new wave of mutants and also has newness going for him.  But which one belongs?

 Battle of the Explosive Babes - Dazzler or Jubilee or Boom Boom?
None of the above.  Personally, I don't have much of an opinion of any of them.
Colorful Transporters - Nightcrawler
I like Blink, but Nightcrawler's German and and climb on things.  I know his powers aren't as vast but I love his fighting style a lot more.
Brooding Dudes - Chamber
Dude, Chamber blew off his face.  That makes him a LOT cooler.  Plus he's British ^^.  Honestly, though I've just liked Chamber a lot longer and sometimes Rictor does things that make me want to slap him.
Bulky Guidos - Strong Guy
Hard one.  When push comes to shove, though, Strong Guy is a lot funnier so he gets the prize.  He's the tank who knows he's a tank and thinks it's funny.
Pretentious and Elf-Like - Northstar
This one wasn't even in the park!  Northstar is way cooler even if he can't stay alive and important for more then ten minutes.  Plus, I like his asymmetrical costume
Teenage Mutant Ninja Geniuses  - Prodigy or Cypher 
Don't care.  Both are boring and just kinda...there.  I like Prodigy's original costumes better, that's about it.
Arrogant Playboys  - Hellion 
Haha, ya, no contest here.  Probably mainly because he's one of my favorite characters.
Masters of Evil - Mastermind or Lady Mastermind 
Don't care...
Unphased Youth - Loa 
Only because I've actually read stuff with her in it.
Firey New Mutant  - Firestar or Magma 
Don't care, I barely know about either of them.
Maiden of Metal  - Mercury 
Husk is cool and all but Mercury is way cooler.  And also one of my favorite characters.  Husk tends to do things a little "slap-worthy" while Mercury  seems a little less distracted by boys most of the time.
Giant-Sized X-Men - Colossus
Legacy or no, Warpath can't measure up to the X-men's Russian metal tank.  Period.
Quickly Catty - Northstar
Again.  Gabriel has never grabbed my attention and Quicksilver needs to re-earn respect in my eyes.  He's kinda...lame at times, and a wimp.
-Virus