Why do red Saber and blue Saber look so alike? On the surface, red Saber (from Fate/Extra) and blue Saber (from Fate/Stay Night) look almost identical. 

Was it ever explained why they look so similar? 

What noticeable differences do the two have, aside from their abilities and attire?
Saber is extremely popular character. They are just "milking franchise" with character that looks similar to the original. And by having little different personality, they make sure they gain interest of more people other than fans of original Saber.