My mares really fussy when it comes to chaff and generally just chucks it everywhere and I got recommended to replace chaff with fast fibre instead and she preferred it as it was nice a sloppy.
I feed supplments, a bit of micronised linseed and speedibeet. I don't use chaff. My horse seems fine with it. Speedibeet is high fibre, low sugar (if using unmolassed) so equally as good as fast fibre i would think.
Depends what you are trying to achieve by feeding it. Speedibeet can send some horses nuts and I believe has more calories than fast fibre which is basically cardboard (so my horse says!).
Some horses seem to prefer a fibrous feed and don't like slop others aren't that keen on chaff and like wet feeds so that could influence what you feed.
Fast fibre is basically sugarbeet and chaff with a few added extras so if you think yours would prefer a wetter feed but want less calories that just speedibeet it could be an option.
But as others have said also depends on what else you are feeding and what you are aiming at weight loss/gain, more/less energy etc