I have not seen a single reason to switch from my Z2 to a Z5: neither in terms of speed nor the specs. The Z2 is still a great well-balanced phone. Z5 is way to expensive for no reason. it offers nothing essentially better that the Z2.
The Z2 is such a beast of a phone, mine is 2 years old and I still can't really find any real reason to upgrade. The Z2 set the bar so high the Z5 needed to be something a lot more than it is. it is better, but I'd rather keep my Z2 and £650
proud to own z2 over 14 months till now, i'm thinking of buying z5 but after watching this video I get back ! no much difference to get z5..z2 still struggle .. wish someone can port camera app of z5 to that of z2..