127 milimeter is in the middle between 50 and 180.
But MF cameras are great with their 80 , 90 mm lenses. You can use 90 mm as a 1 to 1 magnification lens and up to 3 meters range , they are really great. You will want to close and close to the close distance subject when it magnifies .
May be you buy both of them.
Did you look to the lens galleries , do you have diagrams of these lenses ? May be knowledge about lens design would help to you to select one .
Look to the diagrams of zeiss , pentax , kiev , rollei and compare with yours. It would be open new doors to your selection.
I think compressing a distant object in to flat perspective is great fun but your 180 do this.

Edit : Look herehttp://www.flickr.com/photos/nelsonfoto/2499851616/

I think 90 is not sharp and midtones lost , skin texture is lost. Not anatomic quality is presented , even hairs are not sharp.

Look at here :http://www.flickr.com/photos/thomash...7602828405555/
I think 127 is excellent , MY ADVISE is 127.

Umut