Yes, there is a difference mechanically.
Yes, it can be carried without the safety engaged.
I would say based on your question, and not to be presumptuous, is that you want a lot of trigger time with this pistol, or any pistol, you intend on carrying to potentially defend your life, of the life or your loved one.
Know your tool, and know how to manipulate it quickly and effectively.
Due to the nature of the Shield safety, I would agree it is incredibly unlikely to become engaged/disengaged by accident. That said, I would caution you to train as if it is "on", even if you "leave it off all of the time".
I understand there are varying schools of thought here, and to each their own, the old school types like 1911 guys, revolver guys, or the newest school striker guys, safety striker guys, etc, but if a safety is on your weapon, you should be comfortable manipulating it on/off instinctively, period. The best way to establish that is through repetition-- training.
It is an extra step to train.
A Shield safety can be disengaged from the draw, like most other manual, external safeties, but the last thing you want is to pull out a tool that you immediately require in order to protect your life, and no matter how hard you pull on the trigger, nothing happens.
Seconds count