I carry both these phones around.

MP3 playback (internal speakers) -- surprisingly the 3110 C wins in this aspect.

Camera -- The quality on 3110 C really sucks for Video and Image, period.

Durability -- I've dropped my K750i 3 times with its plastic protector attached without any damage. Haven't tried on 3110 C yet but I really like the rubber-based material which covers the entire back plane of the phone. The K750i joystick will detoriate overtime, so will the Select
key. 3100 C is about 6 months newer, never dropped, the keys are HUGE, the "joystick" is actually a button.

Battery Performance -- Nokia 3110 Classic seems to last longer than K750i.

Navigation -- the Gallery/Media folders are rather confusing, there are so many folders for audio, photo and video. K750i folders are more organised. However, 3110 C merges both SMS and MMS in your Inbox, rather than K750i separating them out in 2 different Inbox.

Features -- I'm not a phone freak so I don't use any of those advanced features. However, K750i has this Torchlight feature I cannot live without. For 3110 C, reading (Play) incoming MMS text auto-scrolls but ends halfway. This really irritates me but I guess you can Read the full text separately in a separate screen.

If you ask me to pick a winner, I would still go for K750i :

+ stylish
+ torchlight
+ screen quality
+ shorter height

- shorter battery life
- joystick and key durability issues
- slightly thicker depth