In normal usage, not a lot of difference. Sleeping means that your Mac will be available to you quicker since one keystroke and it's live again rather than having to wait for it to boot up. It won't do any harm to leave it sleeping - you can even set it to wake up at 2.55am so it can run its cron jobs and then go back to sleep.
If you live in an area where there are frequent powercuts, then you may want to shut it down since Minis will need a powercord to sleep (unlike portables).