My old mare has Cushings, so she has to get the low carb senior and a pill for her Cushings. I use Triple Crown Low Carb for her. If your old one is an easy keeper, or has Cushings, the Low Carb option is good. Helps prevent Founder.
I agree with 76 about Purina Senior for the thinner old horses. With poor teeth, add just enough water to see and then let it soak for about 10 minutes (I had to cover it because every fly on the farm loves the smell of it). My first horse lived until 34 on that feed. She ate hers from a big tub. They get so messy and dribble it all over the ground if you feed out of a regular feed bucket.