These are two very different Cichlids.
Discus need 55-75 gallon tall minimum. A mature tank is essential and they're super sensitive to water quality. They're notorious for needing a low pH and many keepers have peat filtration to achieve this. Discus are high maintenance animals often needing bi-weekly water changes. They're best bought from a breeder where you can replicate their tank conditions than a pet store. Contact a local/national Discus Society/Association for more details of local breeders/shows you can go and visit.
Oscars on the other hand are a little less fussy on pH and fairly adaptable. They are however extremely messy and need a 75-100 gallon tank minimum for 1, this tank needs heavy filtration and regular routine maintenance. A lapse in water quality will result in problems like Hole in the Head. They will get 14-16" long and be highly territorial. For tankmates you really need a 120-150 gallon tank minimum and then they must be similar sized and robust like common plecos or black banded leporinus. They will also "redecorate" tanks so decor must be very secure.
It really depends what look you're going for and how much room you have for a tank.
