[size=2]Is there any major difference between the two phones?[/size]
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[size=2]Which one is better?[/size]

SE are the masters of selling the same hardware in a different package.

The hardware on these two phones is the same, the W800i comes with a bigger memory card (though you can buy a bigger one for the K750i if you need to) and a different media player, the K750i has the 'activity menu' the W800i hasn't. One is white and orange, the other is black and/or silver.

The K750i software can be changed for the w800 software.

Personally I'd go for which ever one you liked the look of, taking price into account if you wanted.

hardware wise they are both identical, cosmetically they look diffrent and have diffrent software. The k750i, d750i and w800i are all the same, i have the w800i simply beciase the packaging ware nice and it had a 512mb memory stick.