Sony Ericsson W700i and Z525a just comes with small modifications on the previous W800i and Z520i. W700i is the 8th Walkman phone from SE, it is exactly the same as W800i except that it comes with Gold color, it supports only MP3/AAC ringtone while W800i supports also 40 channels Polyphonic and 256 MB Memory Stick Pro Duo is bundled while W800i comes with 512MB.
For Z525a, it is the enhanced version of Z520i. It is exactly the same as Z530i while it comes with an additional PTT(Push to Talk) function. It will be targeted at US market.