I would like to ask which portable hard-drive from the apple accesorries is the best one? Taking into consideration speed and weight. If it is relevant I have the new Air. Thank you in advance.
Personally I prefer a hard drive enclosure, that way I can easily use any other hard drive when I need to. Also it's easy to upgrade to a larger drive in the future if you need to.
I would seriously hope you are not considering purchasing a hard drive from the Apple Store (unless you're buying it using store credit or a gift card).
You can likely find those hard drives much cheaper elsewhere.
Also, any external hard drive will work with your mac as long as it has the right connectors (USB 3.0 is almost a given these days).
Not sure you understand what a hard drive enclosure is. When you buy a portable HD you are buying a 2.5" hard drive in a case that you can't (not supposed to open). It's in a permanent hard drive enclosure. You can buy a 2.5" enclosure that's the same size as those portable HD's but usually has 2 screws so that you can open it up and replace the HD. They usually run from about $10 to $20 for the enclosure plus the cost of the HD. If you have a 2.5" drive laying around this would even be a better reason to go that route.