Whether sudo preserves the HOME environment variable or sets it to the target user's home directory depends on its configuration (see the manual for details). It's not Ubuntu doing it right and SuSE doing it wrong, or vice versa: there are upsides and downsides to both choices. It's your job as a script writer to cope with both cases. The solution to (1) is to either run sudo -H or pass --homedir to gpg.

However, running gpg as root is definitely the wrong approach. This gives gpg too many privileges, and may the privilege to access ~/.gnupg away from it (e.g. if the user's home directory is on NFS). Run gpg as the user who owns the key, and make it print the data to standard output. Piping into tee is the standard way to output to a file that you need special privileges to write (I have no idea why you consider it “unwieldy”):

gpg -d foo.gpg | sudo tee /run/foo

Whether su or sudo is needed to become root depends on the system administrator's choice. Different users on the same machine may use one or the other. Unless you control the configuration on all machines you'll run your script on, allow for both possibilities (e.g. with an option passed to your script).

If your scripts work on Ubuntu with sudo but fail on other distributions or with su, you may be relying on the environment being (almost) completely reset. That's the default sudo configuration on Ubuntu but other systems may behave differently. Fix your script so that it doesn't depend on the environment being reset.

Whether sudo preserves the HOME environment variable or sets it to the target user's home directory depends on its configuration (see the manual for details). It's not Ubuntu doing it right and SuSE doing it wrong, or vice versa: there are upsides and downsides to both choices. It's your job as a script writer to cope with both cases. The solution to (1) is to either run sudo -H or pass --homedir to gpg.

However, running gpg as root is definitely the wrong approach. This gives gpg too many privileges, and may the privilege to access ~/.gnupg away from it (e.g. if the user's home directory is on NFS). Run gpg as the user who owns the key, and make it print the data to standard output. Piping into tee is the standard way to output to a file that you need special privileges to write (I have no idea why you consider it “unwieldy”):

gpg -d foo.gpg | sudo tee /run/foo

Whether su or sudo is needed to become root depends on the system administrator's choice. Different users on the same machine may use one or the other. Unless you control the configuration on all machines you'll run your script on, allow for both possibilities (e.g. with an option passed to your script).

If your scripts work on Ubuntu with sudo but fail on other distributions or with su, you may be relying on the environment being (almost) completely reset. That's the default sudo configuration on Ubuntu but other systems may behave differently. Fix your script so that it doesn't depend on the environment being reset.