I've seen apps that have both top and bottom bars. In one, the bottom bar buttons are useless. In another, I use them frequently, but would prefer a different interface.
You can do whatever you want, but I don't see the point of having both top and bottom bars when one or the other is sufficient. Avoid overloading your interface with multiple bars, and make sure the buttons are useful and likely to be frequently used.
Why choose to use one or the other? It's not for "visibility". They both stand out, unless the rest of the interface is cluttered. It depends on when you expect users will want to access the buttons. Bottom buttons are easier to tap when using the phone one-handed (both intentionally and accidentally).
Some options to consider:
Use a dropdown nav bar that shows and hides itself when scrolling, as in some mobile browsers.
Use both top and bottom bars, but hide them when they aren't needed, as in some eBook readers.
Move the sidebar/menu button to the bottom bar. Remove the top bar.
Add some buttons to the right side of the top bar. Remove the bottom bar.
Use "tabs" with swiping (or clicking) to move among them.