The best investment to get help in your studies depend on your user profile and what tasks you want to perform on your device. A notebook, however modest it is setup, is a tool for entertainment and productivity. It is clear that productivity is related to tasks that will be performed, and if the hardware configuration is most appropriate for the implementation of programs. For example, if you will only search the internet, writing text, create slideshows and view videos, no need to purchase a notebook with the latest processor with 3D video card. A simpler configuration with dual-core processor, 4GB of RAM and shared video memory is enough.
On the other hand, when you invest in a tablet is even more mobility and extra hours away from the socket. It is also possible to obtain productivity in the gadget, but one must take into consideration which will replace conventional applications programs, or if there is some adapted mobile version.
The best equipment will depend on your personal choice if you own the PC is outdated, in my opinion the best choice is to buy a notebook.