A software wallet on your desktop computer is typically going to be safer than a mobile wallet, assuming your phone has constant internet access. You can use an old phone essentially for cold storage, with nothing on it but your Bitcoin wallet. Then, all you have to do is turn off WiFi when you're not using the wallet. Otherwise, a software wallet is typically safer. Keep in mind, though, that a software wallet on your computer is only as safe as your computer itself. If your computer is vulnerable, so is your Bitcoin.