As a professional gui designer. It really depends on your style.
If you are going for a playful look then I recommend using UI Corners with a radius of around 10-20 pixels. If you are going for a less playful style then a corner of about 2-5 is great. If you are going for a Sci-Fi style then you can use bevel corners via images. If you are going for a sort of sober and solemn style then just remove corners and play with drop shadows to make it more appealing.
I had edited the post to reflect on the game type which is a street racer type game. Think of NFS. I was thinking of cloning the NFS 2015 Theme since its all pretty much just an “electronical/mobile” look (more mobile really).
But I felt that could be a bit too much for some audiences as I am trying to target every platform and age group, and if something is the slightest they’re not used to they will usually leave (most or some anyway but still).
Then I suggest going with sharp/miter corners with a neat looking stroke.
Most people don’t care about GUI design. They just need dopamine and that’s it. Your main focus should be on User Experience. UI design comes after user experience has been thought out properly.