Recently, I got my wife a w700i and got myself a z550i primarily because I wanted a flip phone. I will restrict myself to comparing these two and why I feel w700 is better when compared to z550 as there are some very good reviews for both phones already in mouthshut.
w700 is my first choice because it offers a better value for money due to the following extra accessories/features that it provides:
USB Data cable (none with z550)
256MB Memory card (none with z550)
Walkman Headphones (these are brilliant & must if u listen to music a lot - cost 1500Rs but free with w700)
2MP camera with Flash (1.3MP with z550 and no flash)
Dedicated music keys
Flight mode
Infrared
Price:
w700i: 11200
z550i: 7700
(Global Access, Bangalore)
Summary:
The free accessories and the extra features definitely make the w700 a better value for money as they more than make up for the extra 2500 that you pay.
Hence go in for w700 if you dont mind a slightly chunky candybar phone but if u want something smaller, lighter & dont want all the above features then go in for z550i.
The only advantage with z550 is a faster shutter. w700 takes a few seconds from click to actual capture that it is impossible to capture certain things.