I set the SCD-1 and the 5400 for a side by side comparison and matched levels. On redbook it wasn't even close. The 5400 is much more dynamic. Everything has more rhythmic snap. Also vocal elements recorded in a live space have appreciably more air and presence. It was very palpable on a lot of vocal recordings. Wide, deep soundstage. Very 3-D. In comparison the SCD-1 sounded rather flat and one dimensional.
Playing SACD's, the 5400 presented the music with more air and presence but differences between the units were not as dramatic (as I would expect).
I subsequently had a VSE Uber Clock and Balance Output Board upgrade done the 5400. Everything was taken to another level. Comparable and in some instances superior to vinyl.
I love my SCD-1. It's in my office hooked up to a Chord 64 DAC to some vintage McIntosh electronics, and like the McIntosh gear, it's a classic. The 5400 does not come close to the build quality of tank like SCD-1, but sonically the 5400 is its sonic superior (as it should be after 10 years). Now I suspect if you had a full VSE Level 7 modification done to the SCD-1 it would be competitive with my modded 5400, but barring that, its just not even close.