I'll give you a few reasons why the K750i/ D750i is better than the equivalent Nokia.
The 2MP camera is leagues ahead of anything Nokia are churning out in the same form factor (small candy bar phone). It's actually the best on the market.
The gaming is better due to the ability to use multiple directional keys at the same time.
The screen dumps on the Nokia TFT screens, although they are possibly slightly behind Samsung's.
The MP3 sofware is more functional. You can't even forward or rewind tracks on the 6230.
The battery life is better, possibly upto twice that of the 6230i looking at the sepcs.
All none Nokia's take a bit of getting used to, especially Sony Ericssons. However.. Nothing a weeks work of hammering can't sort.
The only reason to go for a 6230i over the D750i is the familiar interface, which quite frankly isn't good enough to warrant a purchase when both can be picked up for nowt on a new connection.