Android: What is better - multiple activities or switching views manually?
I would say that multiple Activities almost always makes more sense. I just don't think Android is designed for constantly switching its own views - you miss out on so much. You have to implement Back yourself, you don't get any inter-Activity transitions, you have to implement a lot of internal logic to resume an application in the correct state. If you don't partition your app into Activities, it makes it a lot more difficult later on to change the flow of your application. It also results in one mega-Activity that can be a lot harder to handle than a lot of smaller pieces of code.
I have trouble imagining that speed is really an issue; if it is then there's something wrong with the way you're initializing each Activity. For example, I used try to pass Serializable objects between Activities, and that proved to be incredibly slow; when I switched to a faster method of passing objects, the speed of launching Activities increased immensely.
Also, I think it's telling that the Android guidelines for Activity and Task Design don't mention switching Views at all; it's centered around an Activity-as-View design.
I'd like to point out some instances when a single activity might be better design for an Android application that has more than one full screen View:
If the application screens are tightly coupled and share a common Object that they are all operating on. In this case passing around the Object may require a Bundle and can be error prone since there will be copies of it. A good example might be a wizard. Yes you could use static's to access the common Object but static can be dangerous in Android (think configuration changes!)
If you want some really cool animations in between screens. Maybe you want a bird to take off in one screen and land in another screen. Try doing that when each screen is an activity!
On the other hand if one of your screens is designed to be shown by any number of other applications then that screen should be its own Activity.
UPDATE March 2014:
At this point the question should now include the choice of Fragments. I think that Views are probably the least likely choice of the 3: Activity, Fragment, View. If you want to implement screens that make use of the back button then it should be either Activties or Fragments because both handle the back button natively. Fragments will need to be added to the FragmentManager back stack for the back button to work. Managing fragments, dialogs and the back stack can be a bit of an annoyance though!
UPDATE Sept 2018:
Some devs at Google are recommending single activity apps using the new navigation architecture component.