the ux50's color is much techier. i have a ux50 and my gf has ux40... i hope she doesn't read this, but the ux40's color is ugly. for some reason it makes it feel heavier. it looks like metal, maybe that's why. the color of the ux50, however, is light and comforting.
I have a UX40 but I think the UX50 looks better from what I've seen on the web. It looks cooler. I also (DON'T QUOTE ME BUT) heard that the UX50 is scratch-proof Can't say that about the UX40.
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JAmerican
Handspring Treo 90 --> Royal RG135nx --> T-Mobile Sidekick --> T-Mobile Sidekick Color --> PEG-UX40 -- > PEG-UX50--> Samsung T629 (Returned) -- > Palm TX (SOLD)/T-Mobile Sony Ericsson T610(Defective) --> T-Mobile Dash(Broken Screen) --> ASUS EEE 1000H Black/Treo 800w/PEG-VZ90