always go with the SI light. its not alway 15k, the mile gets shoter if you drive in the city, and longer for freeway driving. because the DME gets its input signal from speed sensor and TI signal(average fuel on time). this is how the SI light work. hope this helps.

well on the E36 here the formula for SI lights
e= a(1+t+r)

e=Estimated mileage
a=Actual mileage traveled
t=1 if engine temperature is below operating temperature, else t=0
r=.5 if engine is above 4500 RPM, else r=0


other than this it also computes how many ignition starts have been, since last oil change.
E46 is purely distance travelled.