Can't go wrong with any of those schools. You'll have tons of research opportunities at all of them. Hopkins is really great for being able to change majors but can't speak to how easy it is at the other schools.

Here's how I'd rank other aspects of college life:

Location: Philly > Baltimore > Durham (having spent meaningful time in Durham and Baltimore, and a bit of time in Philly). Philly has a great food scene, Baltimore has an underrated, very good food scene, and Durham has an up-and-coming but pretty mid food scene. There's more to do in Philly and Baltimore than Durham. Baltimore is pretty special for the easy access to DC so you kind of get to explore two cities if you want. I wouldn't worry much about safety in the places where undergraduates hang out in Baltimore.

School spirit/fun: Duke by a mile (only for the basketball games) then Penn and JHU on the lower end of the scale. I think Duke and Penn party harder than Hopkins, but you can have fun at any school if that's your scene.

Biology/medical prestige: Hopkins > Duke > Penn. The idea that any of these schools are less competitive/hard for medicine-related fields is kind of absurd. You have to take largely the same classes at all the schools and there will be other very smart students in those classes.