If you are designing for Android, a full width button similar to the first is okay if you are using a borderless button theme, but if not, it would need to be more like the second. Take a look at the Button design guidelines.
If you are talking about iOS, then it would need to be a button more like the second.
However, neither of them are fantastic as they stand. If you design for Android, you need to make it look more like an android button, and the same goes for iOS. Neither of those two options look much like an Android or iOS button.
iOS button example: