It really depends on what kinds of apps you are making.
- If you are just making a basic app, that you want to look like the standard Apple apps, you shouldn't switch.
- If you really rely on Flash as a base, you should use that.
- If you have little experience with Objective-C and prefer to code in C#/*.NET*, you may go for MonoTouch.
It really depends on your preferations, but if you want the most stable, reliable, and supported platform, you should go with Objective-C.
In my eyes, you should really only be using anything other than the standard Objective-C if it is utterly necessary, as others have pointed out; with Apple, you get the latest API's first, better performance, etc.