well i just wrote two 9 minute long pieces. the loudness gets affected not until the 10 minutes.
the loudness and quality of cut DOES get affected.
The shorter the tune, the more spread out the grooves can be, and the deeper the grooves can be.
Simple.
Unless you don`t use any bass.
Also you get more time at 33, but 45 cuts better, because the wobbles in the groove are spaced out more, once again.
Although I have been told that certian frequencies cut better at 33 than at 45, but we don`t really need to get too deep into this.