The two speakers are very similar. Both use Watkins Dual Drive woofers and the smaller EMIT tweeters, both are rated as reproducing 38-32 kHz +/-3dB, even the non-mirrored boxes are similar in size; Q5 is 26.5"X15"X12" while the RS1.5 is 26.5"X15"X11".
The only real difference is the midrange driver and the crossover. The Q5 uses the 1 1/2" dome crossed at 4 kHz and 600 Hz, but the RS1.5 uses a 5" cone mid that is crossed at 5 kHz and 350 Hz.
The differences will be subtle...