For high work the Xtreme sill with it's lowest row of bristles removed is a good choice. Although, I wouldn't go with flocked if you intend to rinse on- or remove anything other than normal, airborne soiling for that matter. Removing the lower bristle row returns the unit to a more 'brush like' ,meaningful and practical product rather than a bog brush-looking, ineffective, apparent sill cleaning effort. The extra one row of 'effective' bristles does add stability and aids cleaning, making it a better option than the standard Xtreme (it also has better jet positioning over the standard Xtreme). Think of it as a Xtreme/Ultimate hybrid. The lowest row of bristles on the Xtrem sill, for me, is pretty pointless and diminishes an otherwise good brush.
Anything below 30ft I use the Ultimate, a bit weightier than the Xtreme granted, but cleans more efficiently.