http://www.gsmarena.com/compare.php3?idPhone1=6144&idPhone2=6539
http://www.gsmarena.com/sony_xperia_z3-review-1140.php
Spec wise, both phones are very similar except for upgraded cpu and 2gb vs 3gb RAM, for which you won't see any difference in today's apps, but design wise I understand that some issues in the z2 were addressed in the z3, for example, smaller bezels for the same screen size. I would think that in these case sine there is no clear advantage it would depend more on what you want

. If money is no issue, z3 then... probably z2 if it has a neat discount.
Also, I am assuming that 5.2" is what you are looking for considering your 2 options... but might consider the z3 compact, 4.6" sharing the same specs of the z3 except...