Hopefully I can give you some insight on this!
When comparing horse feeds, it is important to think of energy content. Protein is the least efficient energy source for a horse to use, and a mature horse needs only about 10% protein in their diet. Younger horses should have a higher protein diet, because the amino acids that are "protein" act as building blocks for tissues and bones in the developing horse. Feeding a horse a higher than required protein amount (if you are talking about 14%) is not detrimental....your horses' urine will just be a bit stronger smelling due to nitrogen exiting the body.
Fat is the next important thing to look at. Fat is the most efficient source of energy in a horse feed, so the higher the fat content, the higher the energy. Most feeds that use added fat are around 6-8% fat. Adding more fat to a feed creates a higher energy content because the horse is getting more calories.
Fiber is the next thing to look at. Fiber is an important component of horse diets, especially because bacterial fermentation produces volatile fatty acids (VFAs) that fuel the horse. These are broken down slower, and require more energy to convert to energy. A diet higher in fiber is lower in energy.
With this being said, Equistages has less energy that Strategy GX because there is more fiber in Equistages. This necessarily isn't a bad thing. Both feeds are good from what I can tell, but I've never fed Equistages. If your horse is in average work and you want to save some money, go to Equistages. If he starts loosing condition, switch over to Strategy GX to give him some more calories.
Remember that fiber in a horses feed isn't a replacement for long stemmed forages (hay). So be sure to give him approx. 2% of his body weight in forage per day (this is just the average).
Note: I'm leaving out the discussion on vitamin and mineral ratios because in formulated feeds, they are in a good range.
Hope this helps!