If I were taking this route, I would want to work on med surg floor as an LPN. You will gain much needed experience that will help you out when you go for your RN and will have that experience to report on your resume. Many places don't want to hire new grad RN's into a specialty until they have that med surg experience under their belts but you will have the advantage since you have worked it already.
That being said, we have several new students in our second year RN program who are working on their LPN to RN and they come from much vaired experience from Psych hospitals, to a prison nurse to an Army medic to floor nursing on different departments and even an LPN who worked in a doctors office. It seems that the one who worked in the docs office lost some critical skills by not using them on a daily basis and the two that seem to be doing the best are the ones who worked at the hospital. They have also helped us regular RN students immensely and will have a breeze in clinicals as one will be doing clinical on the floor he usually works on!
Best wishes in your career choice!