Strategy is a pelleted feed, but you could feed a sweet feed or even a complete feed though you would need to feed alot more of a complete feed to meet your horse's nutrient requirements. You can definitely choose between lots of different styles of feeds for your horse, but for a hard keeper you want to choose a feed that is designed to provide more energy (which equates to more fuel for the body to work or bulk up) such as feeds for working or performance horses.
The best way to figure out what kind of feed to use in a situation is read the label. The manufacturer tells you what age or work situation the feed is designed for and what is an appropriate amount of feed for your horse's size and workload. Though these amounts on the label are general recommendations and may need to be altered based upon your specific horse---hard keepers will need more, while easy keepers will need less.
Many of the big feed producers like Purina and Nutrena have websites where they can help you select an appropriate feed.