Hey Angelface21
Interesting question! We recently had a very similar discussion in the german forums.
Basically you should always rebuy to the maximum that is possible as long as you give yourself an edge. As long as you have an edge it is always +EV. There are very few exceptions from this, but they are of less importance.
The main advantage of rebuys is that you don't have to pay rake for them (at least not if you play pokerstars.eu/pokerstars.com). Therefore they are cheaper if you compare them to the initial buyin.
Why should you take 2 Rebuys instead of one?
Well - if you have a double stack you will be able to accumulate double the number of chips if you go allin against another deepstacked opponent.
On a side note you should always take the addon - without exception.
Hope I was able to clear things up. If you have further questions feel free to ask!
Regards,
Asaban