Currently I like both but if I could only keep one I’d take the HD700s, the 700s are more focused (the angled drivers are a kin to properly positioned speakers) with details being better defined from top to bottom with a tighter bass (the improved bass on the 712 is only due to the new pads), soundstage has more coherency (again from design) and I love the fit (on most phones it is either hit or miss, and I much prefer the microfiber fabric on the 700s over the velour of the AKGs for long term listening)).
Basically I believe Sennheiser have given us a technically superior headphone with a lot attention to detail with regards to its overall creation in mating the drivers to the cups, baffle and ears and IMHO it works great as opposed to AKG and many other manufacturers who just throw new pads and possible a slightly different driver on a somewhat modified signature structure, think also Grado, which just holds a pair of drivers next to the ears.
The K712s are not bad headphones and I have full confidence in AKGs capsule design, (they are not new kids on the block having produced highly regarded headphones and microphones for many many years), and can be quite satisfying but Sennheiser with their R&D as I mentioned took things much farther (which started with the HD800s but IMO a bit over the top and which probably accounts for their polarizing acceptance), in many ways the 700s with applied trickle down technology are a more successful headphone, but that’s only my opinion and preference
But presently I do like to alternate between the two, the K712s overall presentation is more pedestrian and warmer in comparison to the 700s (which may be why the 700s are not as widely accepted as they are different and an improvement over their HD6xx stalwarts) but in a similar fashion AKG have also “somehow” managed with the K712s in giving us a different presentation, being less bright with much increased (not necessarily improved) bass extension and body over the previous K7XX series but this was achieved by applying more of a band aid approach with new pads (maybe drivers) instead of a completely new overall technical design like the 700s, this was reserved for K812s but again a variation over their classic appearance.
Many say the HD700s were/are way over priced when introduced but were a completely new headphone, which probably initially justified the costs.
To Sennheiser’s chagrin the 700s are not the overall success they may have imagined in the head-fi community, again most likely due to pricing.
They are still on average twice the price of he K712s, $700 vs $350, again IMO worth the difference but only if they satisfy what one wishes from a headphone.
I’m music lover first but still an old stick in the mud audiophile and on that premise the 700s are the more successful headphone in presenting an overall better sonic presentation compared to all the other phones I’ve had in my collection over the years.
Now having said that, if there was not a HD700 the K712s would do just fine and for now I'm keeping both.