Due to the position of a belly piercing, be aware that the skin around this area may be especially sensitive. For most people 9K gold is trouble free, but be aware that it can invoke mild reactions for those with very sensitive skin.
The lower nickel content found in 14K gold improves it's hypoallergenic qualities and it's durability
In the case of 18K gold, as well as being more durable, the inherent higher gold purity means even less nickel content.
So, unless you experience dermatological reactions when wearing other 18K gold jewellery, this will make an ideal choice.
18K white gold also maintains it's silvery appearance for longer than 9K and 14K.