I've only heard the 840C in a friend's system, which is very different from mine (I have Thiel 3.6s, Quicksilver 90 watt monos, and Quicksilver Linestage). As such, I cannot really comment on how it compares to the 5400, which I have owned for about a week now.
My frame of reference is the Rega Saturn I owned for a couple of years prior to the Sony. I recently opted to go with the 5400, mostly because I wanted a player that would do SACD. I am aware the Sony supposedly requires a long break-in time; I got mine lightly used, but I have no idea how many hours are on it.
At this point, the Sony is definitely more revealing of nuances and details in the recordings than the Rega. The Sony does great imaging/soundstaging and has lots of air/space between instruments. I'm definitely hearing more attack on the front end of instruments, as well as subtleties I didn't notice before (e.g., tremolo effects on guitars, doubling of vocals, low-mixed instrumental lines etc.). The 5400 does a great job of sorting out complexities in the mix. The bottom end is also both clear and full.
The Rega Saturn may be a tad warmer sounding, overall, with a bit more "meat on the bones." It has a more organic flavor and a bit less of a "hi-fi" presentation than the Sony 5400. If you like hearing the venue and the fine details, you may prefer the Sony (although you're going to hear "warts" and problems in the recordings, too).
Honestly, it may be easier to forget you're listening to digital on the Rega. It does all of the hi-fi stuff, but with a slightly less analytical approach. Both players are great sounding, but with quite different presentations. For now, I'm really enjoying the Sony 5400.
Note: I've been a drummer for a very long time, so my hearing in the upper mids/lower treble is definitely not what it used to be. Needless to say (I'll say it anyway), my subjective preferences, my room, my gear, and (especially) my ears are not yours. As such, your experiences may vary considerably. That said, I would not hesitate to recommend that you give the Sony a listen in your own rig. I've been enjoying the heck out of mine. Good luck!