Is canned or jarred minced garlic substantially different from fresh garlic?
Yes, it is different. Does it matter? It depends.

If you're going to use garlic in a stew or anything else that would 'dissolve' the regular garlic anyway, it doesn't really matter all that much in my opinion.

If you want to preserve the texture and/or create a more 'urgent' garlic flavour in short-cooked food, I'd go with fresh.

Sidenote: most of the prepared garlic comes with additives, consider if you want to have those as a part of your diet.

As for keeping time once opened: no worries really - bacteria really don't like garlic all that much, so refrigerated you should have no problems hitting 6+ months.