I'm not sure what you mean when you say Costume A and B, so maybe you know this already and this is what you're referring to: Any costume slot you unlock after the first gets access to the full range of costume pieces that any other character gets. The only restrictions are 1) you're stuck with Arachnos weapon options, and 2) Crab Spiders are stuck with the backpack (as are any alternate builds if even one of them is Crab Spider). You can unlock your first extra costume slot via one of the VEAT contacts at level 10.
So you're only stuck with the really limited faceless-footsoldier-of-Arachnos until level 10, then you can make a 2nd costume with all the options other characters have.
I figured that out from some posts here, but didn't know this previously. Oh well, suddenly I have something new to do in CoX. 
Quixotik
"I did not say this. I am not here." -Guild Navigator
although I never use it, I love having the SoA outfit because no other characters in game can get it and it has nifty metallic 2-colored textures.
I do wish male widows got a buttcape though 
and also, I wish we were able to get arbiter outfits more easily(color editing on the cape or something)
Magisterum- 50+3 Fortunata--Virtue
Lukerion- 33 Emp/Rad Defender--Virtue
Noah Heartily- 34 SS/SD brute- Virtue
Mika Heartily- 50+1 Fire/MM blaster-Virtue
Okay, I realize the RP is to belong to Lord Arachnos's army, but there could still be some greater options, like weapons, helmets, bare skin arms, patterns, etc.
The choice of costume A or B or don't play has always left me with the choice, don't play. However, after 7 years I needed something new to do so I chose costume A, but I want more costume options for RP's sake. Actually I would like to take the powerset and make up my own character as usual, but I am sure that ability will never happen.
Quixotik
"I did not say this. I am not here." -Guild Navigator