I use the shortest bar possible for the job.
Only exception being, I use a long light bar
when cutting small brush close to the ground,
this saves bending, and I would be bending a lot cutting whin bushes.
A pole saw would be better as I can hang it on a harness to carry the
weight and get a better cut without jagging myself on the actual bush am cutting, there is often acres of them to cut, they always on bad ground, hills,
stony, wet and unwelcoming, they grow close together, had a hard time
finding this single one to show the thick bottom that needs cut, they
are actually hard wood, heavy and dense, no thicker than 2 inch most being
1" or over when densely packed.
http://ih1.redbubble.net/image.9908524.9989/flat,550x550,075,f.jpg