"Image quality" commonly includes factors such as sharpness, resolution, distortions, aberrations, flaring, ghosting, etc. If all of these are identical between lenses, there would be no difference in image quality. In practice, it's unlikely you would have lenses that are exactly the same except for focal length.

There would also be no significant difference in low-light performance because you have posited that max aperture would be the same.

• The F-stop system already accounts for light lost over distance. With the same F-number, lenses with longer focal lengths have physically larger apertures. (200/2 = 100) > (50/2 = 25).

• There can be light transmission differences among lenses (T-stops). Focal length alone is insufficient to predict the T-stop of a lens.

There are two ways to achieve "same composition":

• Move the 50mm lens closer to the subject. Some details will be relatively closer (noses) to the lens than others (ears). The closer details would be slightly easier to capture than the farther ones. However, it's not necessarily the case that the pores on people's noses are more important than those on their ears. Overall, such differences average out.

  You may also consider the perspective change caused by viewing the subject from different distances to be a factor in "image quality".

• Use both lenses from the same position and crop the image taken with the 50mm lens. This will result in a loss of resolution. Grain or noise may also appear to be increased. Many would consider these to be reduced "image quality". However, the variance between center and edge would be reduced. Some people consider that improved "image quality".

Other factors:

• Camera shake is more likely to affect a 200mm lens than a 50mm lens, even on tripod.
• With great distances (miles/kilometers), the atmosphere itself will affect image quality. For distances typical of portraits, the effect should be insignificant.