On paper, it is widely expected that both the Sony PM-0770 and PM-0771 will be a pair of variants of an unannounced Sony Xperia M2 Aqua. Regardless of the model number that you pick up, it will in all probability feature a level of water resistance, which is currently missing on the original Xperia M2 lacks.
Shared hardware specifications between the two models would be Wi-Fi a/b/g/n, Bluetooth 4.0 LE, ANT+, NFC, and GPS connectivity, alongside an FM radio. The PM-0770, however, will play nice with LTE bands 1, 3, 5, 7, 8, and 20, where as the PM-0771 is compatible with just bands 2, 4, 7, and 17. It does look as though taking the water resistant route is going to be popular with Sony Mobile.