Wow! Are you not a lucky one. You have been given a choice of accommodation! I guess you are a westerner as they are the ones who are given a choice such as yours.
Coming to you query, the chief factor you need to keep in mind is your place of work. While Al Waab is also quite a decent place for staying and is not far from the main center of business, Umm Ghuwailina will be a better choice as it is nearer the main city center and closer to Corniche than Al Waab. Suggest you go for Umm Ghuwailina.