Yes W850i is worth the extra money, i find sony ericsson and nokias to be very easy to use. With the W850i you get A2DP (support for wireless bluetooth headphones), it can more memory up to 8gb cards, it looks way better, keypad is nicely spaced and soft touch, and the back of the slider is stainless steel.
If you are still wondering which to get read these reviews from gsmarena and mobile-review
W850i - http://www.gsmarena.com/sony_ericsson_w850-review-114.php
http://www.mobile-review.com/review/sonyericsson-w850-en.shtml
5300 - http://www.gsmarena.com/nokia_5300-review-117.php
http://www.mobile-review.com/review/nokia-5300-en.shtml