got to agree that soft are best, but do wear quicker!.
Just experiment with different makes and hardness's and plumb for the one you feel most comfortable with...winter or summer.
How many channels you have is down to the individual, I have several different sizes. The 14" one is the most commonly used though, do have an 18" one but hardly use that one, I am at least as quick with the 14" one.
The bigger the channel does not mean the faster you will go either, for the proof of that you only have to witness our own 'Turbo' Terry in action

though it can help on huge plate glass windows...fewer passes etc.
Ian