Having had both a 6230i and a K750i, i would say beyond a doubt that i would go for the K750i every time. I actually have more issuses with my 6230i (even the replacement would turn itself off) but i accept the 6230i has probaly less. I have been a nokia fan since the first early analogue phones nearly ten years ago. I tried SE, moto's, NEC's and samsungs but always went back to nokia's. But the K750i is the first one to have really converted me away. Many of the issues have been sorted on the K750i, such as some operational delays some older SE's.
This is why i prefer it (may i add these are my own opinions, i don't expect everyone will agree so don't give me a fanboy stylie flaming). I have also tried to be fair, i'm not a fanboy and dont card what make my phone is, as long as i like it...
Menu system - once you get used to the difference, the new SE menu system is better
imo. I know the old saying 'if i aint broke, don't fix it' but nokias are begining to seem rather dated and need an update. With all the new features it can take some navaigate down the tree to the option you want. I prefer the K750i menu system ten-fold.
Build quality/style - The 6230i's body is nearly over two years old and looking rather dated. Personally i think its a bit dull too and i think the K750i is far more stylish, but thats competely subjective i realise and not an issue for some people. The 6230i is plastic, the k750i is metal. Also, as with all x-press on nokias, the the 6230i takes on dust and i was forever having to take it appart to clean it out. No such problems with K750i. I also prefer the keypad.
Screen - K750i wins this hands down. But the 6230i is still a large improvement over 6230.
Entertainmant - Far better games engine in the k750i, supporting rather fancy 3d games that laugh in the face of the nokia. But im not much of a mobile gamer myself really so this would not infulence my chioce personally.
Messaging - The email client on the k750i is much better. Texting is much nicer, but actually very simular to nokias anyway and you can set it up how you like it so its almost identical to nokias too.
WAP - Much better brower on the k750i
Camera - K750i is best on the market at the min, no contest there.
MP3 - Better on the K750, but not perfect - Supports proper onboard playlist management and supports drap & dropping of files, unlike the nokias which force you to use the awful buggy Nokia PC suite. SE's suite is no better, but at least you can just use windows explorer. The nokia has to have all the MP3's in the one root folder, which is a real pain. The SE lets you nest as many folders as you like. Also the SE headphones in the box are FAR better than nokia sad effort. But you can buy adaptors - a must for a the nokia
IMO, but you can get by with the SE's.
Another thing, the k750i has a play, pause button on the side. With nokia you have no navigate into the menu system. Handy on my bike and in the gym.
Others featues - The k750i has far better video playback for things like tv episodes etc. Also, the k750i has an incredibly powerful LED torch which i use all the time. Better calender too. The k750i has USB charging too, which is useful in the office. The bluetooth on the K750i is also far more rubust. I never managed to sucesfully transfer a file more than 2mb on my nokia very often, even to other nokias.
Battery - I can't decide which is better really. Out of the box the nokia's tend to last a bit longer but they seem to degrade faster over time than SE's.
Memory card - £240!!! for a 2 gig card??? Where from? Kay's Catalogue?! Also If you shop around you can get 1 gig ones for around £70 (my 512 cost me £25). The memory cards in k750i's are 'hot swapable', i.e. you don't need to turn the phone off. You change the nokia card you have to remove the battery. Also, the card works in PSP's so if you have one that may be a bonus. And thanks to the PSP, memory sticks are coming down more and more.
Granted, the k750i had issues, and in many repects the 6230i is a more rubust firmware. But i have no problems since changing it. You can also update the firmware on SE's website using the cable in the box. Can you do that with nokia? I am sure they have been many more faulty k750i's, i know about 10 people who have them and one was faulty. But once you have a working one i think most people would prefer them.
Granted, many of the above features are not important to everyone. But if you are really interested the MP3, Photos and more up to date fautures, they K750i is the obvious chioce by a considerable margin. If i was a business user who just made calls/texts i would go for the 6230i over the K750i, but only by a small margin. But then if i was that kind of user i would would not go for either
tbh.
One other thing, the K750i will probably have a higher resale value in 12 months time when you upgrade.
But at the end of the day, i prefer the k750i for the above reasons. You may not like it. It's totally subjective and you should get a 14 day trial on any contract. Try both then choose