Our senior Belgian had some weight to gain when we first got him (like about 300 lbs!) and he hates beet pulp, picked through supplements, etc. Here's what worked best for us: first since he was already on Purina senior we added Calf Manna supplement and extruded soybean meal. After a few months we slowly switched him over to the Calf Manna equine senior formula and cut out the other two things.
He's now at optimal weight, glossy, and has tremendous hoof growth and his mane and tail no longer feel like dry straw.
It took a year, but as long as you go slow, weigh your feed/hay rather than feed "by the flake" you should see improvement.
These complete feeds can work out really expensive, sometimes its better to provide a good mineral lick or supplement and add beet pulp to what you're already feeding for extra weight gain, its high in fibre and calcium plus has the moisture content
There is always the ulcer and teeth factor?? But to answer the beet pulp or rice bran question I would recommend beet pulp it is much easier (less messy) to feed in large ammounts and the horses seem to like it a lot more, I had a gelding with sand colic that dropped a lot of weight the beet pulp seemed to help a lot.
They recommend soaking beet pulp for atleast 12 hours I ALWAYS SOAK MINE FOR 24 JUST TO BE SAFE, as like most people I am always a little paranoid when feeding beet pulp.