How important is this difference in the final sound quality?
Not as important as how they sound to you. Get the one which sounds better.
The impedance of 2.3 Ohms is a wee bit on the low side....this would concern me as it makes these speakers in the "harder to drive" category. Remember that this speaker will take nearly FOUR times the current of an 8 ohm load and this will definitely restrict your headroom (the dynamics your amp can handle where the amp remains below clipping). A massive amp and the more efficient speaker may help in this case.