Sony Xperia Z3+ is less slippery than the Xperia Z3
The back of the Sony Xperia Z3 is extremely slippery due to the scratch resistant tempered glass. In short, it slips and slides where other phones would not, so you've been taking extra care to place it away from edges. Nevertheless, Sony appears to have improved handling in the Sony Xperia Z3+.
The Xperia Z3+ edge has a slightly raised edge versus the predecessor, Z3. This small change improves grip.