Hi Ken,
Naturally the best option would be if you could hear these speakers in the same room, side by side (properly calibrated of course). Otherwise you may not believe the difference there truly is.

A few spec numbers won't tell the whole truth, not even the half of it. When looking at these two speakers more closely, you can notice than there is much more than meets the eye.
[*:2r5960wi]The 312 is a 3-way speaker with a Genelec-designed and made 5" midrange driver - basically one of the best midranges money can buy, bar none.[/*:m:2r5960wi]
[*:2r5960wi]The 312's cabinet is much larger both in internal volume and in front plane's area, which means that the low frequency efficiency is greater and that the directivity of the mid bass frequencies is greater.[/*:m:2r5960wi]
[*:2r5960wi]The 312 has a very high output and good sounding 12" woofer with a 3" voice coil while the 210 has a lower performance 10" woofer with a 1.6" voice coil. This combined with the larger internal volume, larger reflex ports and larger frontal plane area results in superior low and mid bass performance.[/*:m:2r5960wi]
[*:2r5960wi]The 312 has a much larger and deeper DCW (naturally for both mid and tweeter) which results in greater directivity in the mid and high frequencies. This means that you will get more clean direct sound to your ears instead of the nasty room reflections. Larger DCW also boosts the efficiency of the mid and tweeter, which means that the overall max SPL capability is considerably higher (124 dB vs. 129 dB for a pair at 2 meters with music material) [/*:m:2r5960wi]
[*:2r5960wi]The 312's amplifier has much more room eq controls (DIP switches) than the 210, which means that you can calibrate the speaker much more accurately. [/*:m:2r5960wi]
So to sum up it shortly - the 312 is a completely different animal than the 210. It has much higher sound quality and also maximum SPL capability. You have to hear to believe it.