I've never used Sketchup, but awhile back I was Involved in a 12 day contest. In which you were to create 4 Models... A Character, a Vehicle, a Building, and a control (A randomly selected object which everyone competing would have to model)

A Chose a Dalec Human, A Tank, The house from Amityville Horror, and the control was space age chair

I was a able to complete the The First 2, and the last 1, The house on the other hand, was just too complex to complete in the time given.

It was fun!

Anyway, I believe that each package would have It's strengths, and weaknesses, and each would be faster/ slower at certain things accordingly.
Also the modeler Him/her self would be a variable in it all.