it really depends on what you want to do with the phone. I have the k750 now, and primarily like it over the n70 because I can use the buttons. Silly I know, but that is a big thing.
I miss the smartphone capability of S60 something awful though, and I can't find any Java PIM programs to augment the built in PIM functions of the Sony. I am finding I have to rely on Outlook more an more (which I hate, because I don't like Outlook).
So, I figure, if you are particular about how calendars, to do lists, and project management tasks are run (and you can use the keypad) get an n70. If you just want a phone that can sync to Outlook while having a pretty cool camera and music software then get the k750.