For a planted Discus tank, I would use nothing but external canisters, Eheim would be my personal choice.
Discus do not like strong currents, so setting the intakes and returns is imporant for them - you want good flow throughout the tank for the plants and to to avoid "dead" areas, but you do not want to force the fish into high local current situations. This may mean some DIY modification of the provided spraybar returns. Consider during setup taking some time to play with this for future long-term payoff. I routinely modify my spraybars to "tune the current" for the fish. The simplest tuning is just to enlarge each hole one drill bit size - larger holes gives lower pressure through the opening and lower velocity at the bar, providing less current for the same or slightly increased total flow volume. If you want to go first class, hook up the canister on the half-filled tank and lift the spraybar return carefully horizonal and levelly above the water surface - note the difference in the spary arcs from the bar. If you enlarge the most distal holes a couple of bit sizes, the central ones one bit size, and leave the the initial ones as is, you will even out the flow a bit over the length of the bar.
Further, I would use at least two units for the filtration, and for me they would not be the same. For a 90 I would likely use a model 2226 (or 2228) Professional as mechanical-only, that is, all mechanical filtration (Ehfimech bottom, sponges above). this unit would be rinsed as often as I was willing to do it - weekly ideal, bi-weekly good, monthly at least. The other unit would be biofiltration-only* and stocked with either Dupla Minikaskade bioballs or Eheim's Ehfimech (which they sell as a crude mechanical medium, but it does serve nicely as a biomedium), with a FiterMaxIII prefilter replacing the intake strainer. The bio-only canister would have its prefilter rinsed at least weekly, the canister itself backflushed by reverse siphoning tank water in from the return side and out through the intake every one to two months as part of a routine water change. Every six to twelve months this unit would be opened to clean the impeller and its housing.
*
http://www.thepufferforum.com/articles/water/canister.html
The two sparypars I would likely place initially at least along the rear of the tank (back wall) with the openings vertically aligned directed straight up the back glass, about 1/2 suction cup diameter above the substrate. The two water return lines would be tank center back with 90-degree elbows to the spraybars. The intakes would be at each back corner, one extreme left, the other extreme right. The prefilter-equipped intake will need a bit more space than the strainer-only unit, but still is easily concealed by plants. If your fish spawn, use prefilters on both intakes.
Multiple other configurations are of course possible, but that would be my choice.
