Glad you're interested in aerospace.
As for what to major in for undergrad, it really depends on what you're interested in. You should be able to go onto a Masters in aerospace with either. Though with a background in electrical engineering, an aerospace Masters that's maybe more focused on avionics may be easier.
If aerospace as a whole is what you're really interested in, then Mechanical Engineering is the way to go as aerospace engineering is essentially a specialization of Mechanical engineering. In fact, in many universities they are the same department. At my undergrad university, the first two years of aerospace and mechanical engineering were identical and so was much of the third year. Some schools which offer only mechanical engineering do offer elective classes that are more related to aerospace. So you could look into this. But in general mechanical engineering is broader than aerospace engineering but you study the same disciplines such as solid mechanics, fluid mechanics, thermodynamics, materials science, control systems, etc.
You could try talking to a mechanical engineering professor at your school to see what elective courses may be more suited to your interests in aerospace engineering.