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That is a very difficult question. The answer is going to be highly subjective depending on who you talk to. The true answer is that they can both be classed as "best", it really depends on what you want out of the handset.

For example, the camera function; the 2Mpixel SE W900i is much better than even the higher Mpixel N80. The Music Player (Walkman) function is also much better in the SE than the typical Nokia. But these are Sony specialities (Walkman and Cybershot) carried into some of their latest 3G handsets.

However, it may be that in terms of application support the Symbian-based Nokia (i.e. 6680, N70, N80) are more advanced and therefore "better" than the SE's proprietary OS.

Other aspects of the handsets are more governed by international technical standards such as Video and Voice which are on a par with both.

Good luck with your search for the Holy Grail