It's a very personal decision. There's no point reading a book in any language if you don't enjoy it. If you hate sci-fi, for example, in your own language, then reading it in English won't make you enjoy it any more even if it's great for vocabulary (I'm not saying that it is!)
Have you enjoyed reading any books which have been translated into your own language, but which were originally written in English? If so, why don't you get hold of the English language version. The fact that you have already read the book is not a problem. In fact, it may help because at least you will remember (hopefully) the basics of the storyline but you will still get plenty of vocabulary and grammar constructions from the original.
Student or Learner