Better how? I have my Welsh D on Fast Fibre mixed with chaff and oil....I like it as it doesn't fizz her up and is ready in seconds. It's not really conditioning though so if she starts dropping over winter I may substitute or add in something else.
Depends what you want it for. Speedibeet is straight unmolassed beet and is relatively high in calories at 12 MJDE/kg (similar level to Spillers conditioning cubes, dry weight) so will tend to put weight on if fed in significant quantities.
Fast Fibre is unmolassed beet mixed with ground straw and some other ingredients. As straw is fairly indigestible this is a filling feed but low in calories at 8 MJDE/kg, so useful for dieting equines!
I have used both, fast fibre is easier as it soaks up so quick, but I feed speedibeet now as it lasts longer (soaks up more water), and it doesn't fizz mine up or make any noticeable difference to their weight, I feed a lot of it too!