Keeping a long-term and healthy fully planted tank takes a certain level of expertise. This is gained by lots of research, trial and error. Understanding Co2, ferts and water changes is a totally different animal when keeping a glass box with plants. On the other hand, understanding discus, ph, kh, gh, nitrite, nitrate, amm, etc can also be mind numbing. I have done both but never the expert at either. I have killed plants and I have killed discus due to mistakes and lack of experience so I would never recommend someone jump in to either hobby without doing a great deal of research and then do some more. The requirements for planted tanks and the requirements to grow young discus don't match up well so I wouldn't do it if your goal is to grow large discus. Now, if you have mastered the planted tank, there is nothing wrong with keeping adults as their care is not as critical compared to young discus.