It's all down to personal preference. Try both and see which you prefer.
Personally, I have never used a sill brush. I just use an ordinary Gardiners brush. The new 90 degree sockets and quick release angle adapters are set at the perfect angle for most first floors providing you can stand back a comfortable distance which you should be anyway to avoid neck and eye strain, let alone getting wet.
You may have to change the angle down slightly if working in a confined space. ie a lane or somewhere to clean the glass and sills correctly. Also to stop the water running down the pole and getting your hands wet.
The brush should hit the glass square on.