Is canned or jarred minced garlic substantially different from fresh garlic? I am always in favor of fresh ingredients when possible. I recently discovered that minced (and crushed and chopped) garlic is available in very inexpensive jars in the produce section of the grocery store. I've always bought garlic and chopped it for a given meal, but I wonder if such jars of prepared garlic are worthwhile.

Would purchasing prepared garlic in a jar be a time saver in some situations, or is the quality reduced such that it is not recommended?

As a side question, does minced garlic in a jar keep for very long once opened?
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.