I'm not a huge fan of purina--they're overpriced, and often want you to feed way too much in order to get the required nutrients (they wanted me to feed my weanling 15 pounds a day. ...not happening! I went to a more concentrated feed). Purina also has a history of other 'things' getting into their horse feeds.
I've never heard of Tiz Whiz, hehe.
I would go to your local feed store, and see what they have to offer. I feed my horses one step up from the cheapest horse feed, and they're shiny, healthy, and have plenty of energy when it comes down to show season. Overpriced feeds are just capitalizing on a market of people who seem to think that their horses aren't getting enough nutrients/energy from pasture, hay and 'normal' feeds. I say they are silly. :P Usually, almost any 10/12% pelleted feed will work.
I've seen old trainers just feed corn and have ridiculously good results--but it's up to you and the individual horse, of course ;).
But yes, pelleted feed would be the kind I'm talking about. If you are going to ride her a lot more in the summer, I would give her a maintenance pellet for the winter (which my boys are on--it's low protein, so they don't get excess energy), and then wean her over in the summer if she needs it—some horses don’t. My show horse gets put on a 12% show feed just to give him that extra protein he needs to help rebuild his muscles, but he’s worked 6 times a week and shown on the weekends. So, it’s relative. ;) Even if you’re riding every day, you still might not be doing enough to really tax her system—it depends on the type of work.