Digital cable length- SPDIF vs AES/EBU- 1.5 meter for either?

It depends on the transition time.  1m might be perfectly fine with transports that has 15ns transition times.  In addition, reflections appear only on impedance boundaries.  With a perfect match (rare) there will be no reflections.  Very short cables, under a foot, will also work fine.  Rule of >1.5m is just for general case of typical transport.

Jitter is basically an added noise. Music free of noise might sound clinical and sterile at first. One person even posted, after listening to jitter supressing DAC, that he preferred all instruments "together" (sound blob) instead of hearing individual instruments.  Also, music with added noise can sound more dynamic - like distorted guitar vs clean Jazz guitar.  I believe that jitter destroys everything, but we tolerate it since we got used to it.  

Assuming 25ns transition time (typical transport) and about 5ns/m signal speed (typical cable) reflection in 1.5m cable will come 15ns later (after of the beginning of transition) just missing threshold point around 12.5ns (deforming transition by adding to original signal above threshold point). Engineers tried to predict it making even special tools for that (Bergeron Diagrams) but different transports have different (unknown) slew rates while speed of signal in the cable is dielectric dependent. It is all guessing game. System with very short transitions (few ns) will be less sensitive to the first reflection but will produce much stronger reflections on smaller impedance boundaries. We can only say, that there is a chance that 1.5m or 2m cable might be better than 1m cable (exactly opposite to ICs or speaker cables)