So, would you just leave it in M resolution and let it shoot with EXR all the time or would you recommend shooting normally in hi res until the situation calls for EXR?
The X10 is a technically better camera than the P&S sensor HS20/F550, so shooting in 12 MP resolution does often make sense IMHO.
EXR SN makes sense with really bad lighting, you can force it in PASM by setting M resolution and DR100%, so you will lose quite some dynamic range. This means that you have to expose very carefully, often with manual negative EV compensation in order to preserve highlights. Dynmic range suffers at high ISO setting, but OTOH, EXR SN is only useful at high ISO settings with plenty of noise, so two bad things are coming together here, hence the need for careful exposure in order to get clean and detailed low light shots.
However, if you want to use a (fill-in) flash, I'd rather set the X10 to L resolution, since EXR DR doesn't work with flash anyway, but still, the camera usually benefits from some kind of dynamic range expansion through tonal DR compression/expansion. Obviously, conventional tonal DR expansion also makes the tiny flash more powerful in a relative way, as the camera is underexposing by 1 or 2 stops. ISO 400 in conjunction with conventional DR400 is actually a ISO 100 shot with -2 EV exposure compensation. Hence noise is not a problem in the RAW (ISO 100 is pretty much noise free), it will however become a problem in the tonally corrected/expanded JPEG, especially in the shadow areas of the shot. So it will also depend on what you are shooting. In any case, filler flash and conventional dynamic range expansion (DR Auto) do work very well together, just like they do with the X100 (which doesn't have EXR).
For extra dynamic range with no flash, it depends on the situation. If you need higher ISO (like 400) and faster shutter speed, use L resolution and conventional DR expansion. For low ISO shots of non-moving subjects, EXR DR will probably be better most of the time. Personally, I would not use EXR DR with an ISO setting higher than 400. Since EXR DR is basically taking two shots with different shutter speeds at the same time, there might also be some strange effects with moving subjects and longer exposure times.