it usually goes down to personal preference, i have both, flagged/flocked feels better on leaded and georgian, unflagged/mono on standard windows.
Flocked will hold the dirt more in the brush, so they are not suitable for rinsing on the glass.
You really need to get one of each and decide for yourself, they will both clean the glass fine, i think a flocked will clean better on a first clean.
You wont get a definitive answer because everybody will have there own preference as to what they like better.