I agree on the tank space thing, you can do apistos in a smallish tank.
Otherwise... I successfully did them together, same tank, for a while. So it's not necessarily either or, unless budget or tank space (or preference) says so. At least for me, while I did enjoy the variety of apisto species, and some of them are quite attractive, there's just no real comparison. For me discus are just a more striking, more impressive fish, with a lot more presence. Discus can also be very interactive with you-- when they're happy and secure, that is.
I know there are plenty of apisto devotees out there, but for me I remember doing discus, the tanks were discus tanks, I think about doing discus again someday. The apistos were an interesting little fish, but like some of the other fish I kept with them they were simply a nice addition to my discus tanks.
On the other side of the ledger, apistos are relatively straightforward. There's not all the debate, or so much conflicting advice, over this, that, and everything else about keeping them, like there can be with discus. There's a lot of advice out there on discus keeping, that while I won't go so far as to say it's wrong, is either not the only way to do it or is simply not necessary in keeping them... or only becomes necessary with a particular approach to feeding and keeping them.
