Normal bog standard sugar beet (eg Supabeet) is molassed and has a sugar content of 20% and a calorie count of about 12.5 MJDE/kg.
Speedibeet is an unmolassed sugar beet with sugar content of 5% and a calorie count of 12.4 MJDE/kg.
Fibrebeet is a mixture of unmolassed sugar beet and alfalfa with a calorie count of 11 MJDE/kg.
I would choose either Fibrebeet or Speedibeet out of those because of the significantly lower sugar content. Speedibeet is the higher in calories if you are looking to keep weight on.
I've managed to find Equi-beet in my local feed merchants which is 5% sugar and half the price of Speedi-beet. I feed it to my good doers all year round to hold their supplements in and at £5 a bag it's better value than Speedi-beet. It needs soaking for 24 hours though.