What should I do about this situation? Any advice is welcome!?

I currently work within a well respected organisation, albeit in a lowly/entry level position. I have aspirations to one day qualify as a lawyer (I attend law school part time) but I am keen to progress my career in the right direction now. After many months of networking, negotiation and persevering, I seem to have managed to have got my managers on board with a Personal Development Plan for advancing my career at the organisation, with a view to working in the legal team within the next 2-3 years time. They were reluctant to get involved and ist in my career development plans at first but I think I managed to get some of their support after it became apparent that I was seriously considering leaving the organisation. However, a new position which would be excellent for my development, which pays a lot more, (at another similar organisation to my current work place) has come up and I am torn between being loyal to my organisation and leaving for this excellent development/career opportunity at this new organisation. Some of my friends have said that I should go for it, as I need to look out for myself when it comes to career matters. They may be right, as my managers (bar one) have shown little or no interest in helping me develop my career. However, for some reason, I feel I would be letting people down if I upped and left after my personal development plan has been approved by managers. Should I look out for myself and take this new opportunity or should I exercise patience and wait for my personal development to take shape at my current organisation? Any advice is welcome.