I would add plants to the display tank if you could just because I like the look of plants in a tank. If you don't want to mess with the upkeep you could just get floating plants like anacharis/hornwort/cabomba and it would probably be easier to clean up decayed plant matter in the sump so you wouldn't have to mess with uprooting them every time you wanted to gravel vac properly. I'm about to make a sump with plants inside, but only because I have axolotl's, and they're light sensitive so I can't have bright lights in my display.
If you havn't used plants at all, they are amazing. I'm so surprised how much they help my water quality. I've heard a few people claim plants havn't had good results for maintaining water quality, but they usually only put a few plants in they're tank and expected a miracle. If you have a heavily planted tank you will definatly notice a change in your parameters