Is it better to use the systemd service unit's Restart & RestartSec, or use a systemd timer unit?

I recommend to use a process manager to do your job. Luckily you have the best process manager ever on board, systemd. In my eyes it is about semantics, even if both solutions are working for your case.

• Use timer units for cron-like schedules you are talking about.
• Use restart statements to specify a restart policy of a task that ends unexpected. That means you even can define a restart policy for your timer units, in case they failed. Keep in mind that restarting units quickly enter failed states, because they quickly reach a holdoff timeout, if you don't define a RestartSec greater than 1.