The only differences are that the 8310 (in either crimson or titanium) additionally has 1) integrated GPS, 2) OS 4.2.2.170, and 3) new colors.

In general I'm not a fan of red phones, but I seriously considered this one because with a nice black skin or case, the only things you'll see besides the screen are the facing around the screen (black on the crimson curve), the keys (also black on the crimson curve), and the bits of casing in-between the keys (crimson on the crimson curve). So, I soon realized that hey ... I can take this red phone and turn it into an almost all black phone with some crimson trim in-between the keys ... and it suddenly became quite appealing to me.

In the end I settled on the titanium (grey) which has a grey facing and grey in-between the keys, and light grey keys. All in all, I'm not really sure which looks more dark, but I definitely wouldn't mind eventually putting that black facing and black keyboard in my titanium (grey) 8310.