That's like comparing apples and meat.
W810i: Very voice and image focused device, terrific reception and call quality, 2MP camera, great music sound quality but doesn't play any DRM songs. No real Web browser - everything would be a compromise - no high-speed anything, extremely basic email. A great cameraphone.
Cingular 8125: Very data focused device, mediocre reception, a bit clunky to hold up to your head, but a real handheld computer for Web browsing, surfing with Wi-Fi, email, loading thousands of software programs onto, reading and editing attachments, etc. Do you need that kind of power, and are you willing to give up size and reception quality for that kind of power?
W810i review:
http://www.pcmag.com/article2/0,1895,1950620,00.asp
8125 review:
http://www.pcmag.com/article2/0,1759,1940691,00.asp
