Both are dissociatives, thats the only similarity they share.
DXM, because it fucks around with so many parts of the brain, the effects can vary alot.
I wouldnt even compare it to ketamine unless your talking about a heavy 3-4th p trip.
Depending on the dose and the method in which the DXM is administered(Gel cap, Free base, Syrup, Delsym) you could get such a wide verity of effects, from stimulating to sedating, from trippy OEVisuals to wierd and dissociating CEVisuals.
If I took 200mgs in the form of free base, I would have a long lasting yet stimulating high with a touch of psychedelia. If I took 400-800mgs in the form of syrup Id get a short but very dissociating trip. If I took say 300mgs of dxm in any form with a dose of CPM (the infamous anti histamine that's in Coriciden CC) I would get an ultra long not very dissociative(because the lack of DXO) sigma overloaded trip that can be really intense...