Like ponysize said, Pennfield is a far better feed quality-wise over Purina any day. Pennfield has a fixed formula and use premium ingredients, whereas Purina just uses whatever is cheapest at the time and the feed formula can change significantly from one lot to the next.
But to break it down: (and I'm not sure without checking what the starch content is, unfortunately)
Pennfield's Senior has 14% protein, 10% fat and 10% fiber.
Purina Senior has 14% protein, 6% fat (the Senior healthy Edge has 10%) and 18% fiber.
So you'd probably need to add a fiber source with the Pennfield; beet pulp is a good choice. Yes, you come out better on the fiber with the Purina, and it's a cheaper product than Pennfield. But it's also "cheaper" on quality. I would avoid any and all Purina products if at all possible.
Well I am feeding Purina Senior Healthy edge right now and have been for 6 months thanks to coupons that saved me a ton of money! Was feeding TC Senior before. Honestly my horses look the same on the purina, still have glow and energy, I just don't see a difference. While I can get coupons I will continue feeding the purina. When the coupon well runs dry then I will buy the TC senior because I do believe they use better quality ingredients, but not so much better that I can see a diff in my horses. So basically the purina is available at my local feed store, it is cheaper adn I don't have to drive an hour for it. I've been happy so far.