Both may be preferable as they provide different garlic flavours.
- Fresh garlic is sharp, and has bite. It tastes fresh, and works especially well with other bold flavours and fresh ingredients.
- Granulated (roasted) garlic is garlicy, but nowhere near as sharp. It tastes more like the prepared garlic flavour we've all become accustomed to. It's a mellower, deeper flavour.
- Roasted fresh garlic is garlicy, mellow, and deeper than either granulated or fresh garlic. It's brilliant in smooth things like soup or mashed potatoes, and is also a much deeper flavour on a pizza with other roasted things.
One of the coolest things about these different forms of garlic is that you can use them together to pack more punch. Not only can you combine them, but you can add them at different times in a preparation to layer the goodness in interesting ways.
"Preferable" is all about taste, your (and your guests') preferences, and depth.
When I make pizza, for example, I use both marinated garlic and chopped fresh garlic. Sometimes I'll even add roasted garlic over the sauce, especially if the pizza has sun dried tomatoes.
We had a very similar question about making your own garlic oil here:
Botulism, Garlic, Cold pressed Olive oil and mason jars
Oil is effectively an anaerobic environment, which promotes the growth of the bacteria and spores responsible for botulism.
However, these types of risks are common in home canning. Major factories producing canned or bottled goods are going to want to protect themselves against millions of lawsuits, so they are going to take steps to pasteurize (for lack of a better word) the foods before packaging them. Commercially-packaged infused oils have probably either been heated, acidified, or filtered to guarantee food safety.
On the other hand, the linked question above demonstrates that there are people out there who aren't taking the appropriate precautionary measures. If this oil came from a well-known company with a good track record then I would not worry (and you can always contact them if you are worried); however, if it came from somebody you've never heard of, and can't find any reliable information on - i.e. somebody who's making this stuff from home - then I might be a little more concerned.
If you trust the source, then trust the oil. Otherwise, you might want to try sterilizing it yourself; you need to either heat it to 250° F (121° C) or expose it to acid conditions. Neither of those things are really desirable; heating it can taint the flavour or even burn the oil depending on what kind of oil it is, and if you want to take the acid route, you basically need to to pickle it. But there you have it, that's what you can do if you don't trust the safety.
Best Answer
Botulinum bacteria require a moist environment to germinate, reproduce, and produce botulinum toxin. Therefore, there is no risk of botulism from eating fully dried ingredients (unless they already contained botulinum toxin, which would not occur in a normal processing pipeline).
It is absolutely possible to acquire a foodborne illness from uncooked dried ingredients. For instance, there have been various cases of E. coli poisoning from uncooked dry flour, and salmonella has been found in dried herbs and spices. So it's not safe in an absolute sense. But there's no reason to worry any more about sprinkling powdered garlic on your food, than about sprinkling on, say, ground pepper.