uname is for returning actual linux kernel information
/etc/issue is distro-specific
/proc/version returns a useful combination of distro/kernel information (usually... sometimes it's just the kernel info)
/proc is a good place to look for accurate information about the system in general. For example, I usually `cat /proc/acpi/battery/BAT0/state' when I'm not in a desktop environment and I'm wondering how much battery life I have left, even though there are tools for this.