It sounds like you have a good chance at getting a job on the peds med-surg floor anyway, since you work there currently and will be going a preceptorship in peds no matter what. If you would like to work in oncology, it might be a good idea to do your preceptorship there to get a feel for what oncology nursing is like and to make sure you will like it.
In oncology, you'll obviously be giving a lot of chemo, blood transfusions, anti-nausea meds, and other meds to manage chemo side effects. Some of the kids will have other problems related to their cancer, such as ventricular drains, respiratory issues, surgical wounds, infection, colostomies etc. However, the main part of your experience will be related to chemotherapy and blood transfusions. Since you'll only be a student, you probably won't be able to do either of those things, but just observe the RN. But if you want to work on this floor after you graduate, doing this preceptorship will probably be your best shot.
Peds med-surg would be the better choice for a well rounded clinical experience. You'll probably see a few children who have cancer, but they are more likely to be newly diagnosed. You'll see respiratory illnesses, post-operative surgeries, chronic illnesses, diabetes, acute injuries, siezures, the works. You'll care for patients with tracheostomies, G-tubes, possibly colostomies and chest tubes. I think this would be the better choice if you're looking for a place where you can work semi-independently and learn a variety of skills.