The N70 is an S60 2nd Edition device and the 3250 is an S60 3rd Edition device. All S60 3rd Ed. devices need new apps and won't run apps written for older versions of S60 (except for Java programs, .jar files).
So which one is better? I mean n 70 got two cameras and 3250 got a very good music player.so wich one is better at all?
Depends what you want from a phone.... The 3rd Edition you find on a 3250 is a newer Symbian OS and has nicer fonts and looks better and is a lot cleaner. The 2nd Edition is older but because of that it has a lot more applications available.
And if you install the N70 OS onto a 3250 it's not gonna change much, you still won't have 2 cameras