Sci 25 will definitely help prepare you for the NLN. Though you really can't compare the difficulty of Bio 11 and Sci 25; they are really 2 separate animals. If you have had little or no experience with physics or chemistry before then this course will be a challenge.
If you are seriously considering transferring to City Tech what I suggest is to register for Sci 25 and feel it out. If you believe that it is not worth your time there is a few week window where you can withdraw from it and not have it impact your GPA.
However, Sci 25 is probably the only preclinical nursing class that will have the majority of perspective nursing students in it so you might meet someone that will help with your decision easier whether to stay or not.
Whether or not the nursing dept prefers Kingsborough students over transfer I can't say. Even if they did I think they have a few hundred applicants from Kingsborough itself so you still have to do well in your courses. I have not applied to any other nursing programs
as I am sure I will be accepted to this one.
Best of luck!