A quick test with my son has confirmed there is a difference, and it's not just in terms of light transmission. I can't say which lens is better, so stay tuned for my review...it should be up sometime this weekend.
Here's a little preview I whipped up...I'll leave you guessing which 105 I used. All I can say is the people who say each lens serves a different purpose are spot on. Those who say there's little difference have clearly not played enough with both of them to know.
Which is sharpest?...you'll find out later. Which produces better bokeh?...you'll soon find out.
A big thank you to my big little guy for helping me out. It's not easy for a 3 year old to stand still so dad can have some fun with old lenses. As such my wife has volunteered herself and our other son to model for the actual testing of the lenses.