Signup and Login button position in mobile view Left - Right or Top - Bottom , which one is better?
Top - bottom
The buttons are easier to click because of the larger size compared to when you put them left to right. See image below.
As Hoshts points out, having the larger buttons also helps in other languages. English is a relative 'short' language, but you might need the space in longer languages (for example, Dutch or German).
I would suggest it should be top, bottom and more towards the bottom center as user is more oftenly trained to click on home button and user has kind of adjusted to this position.
It will also provide room to accommodate descriptive labels inside buttons.
Login should be at the right side and SignUp should be in the left side.The reason is, User will do Sign up only one time then after every time they need to do Login. So, give them easy access to Login button in such a way so that every time they don't need to pay attention before they click.