It will be interesting if after your screening at Emirates & Alpha, you write back to us here... compare and contrast the two experiences and your overall impressions.
Sometimes its difficult to make a choice when you don't have all the critical facts. Pprune helps with this. You are doing a great job so far doing the research and asking the right questions.
Be sure to ask the real, total costs from beginning to end including the 1500 hours + the MPL, and ask for training dates, e.g., what month/year will you start your line training... and what if there are significant delays? Ask Emirates if they would hire you if you had an MPL + 750 to 1500 hours of line training... as a back-up plan if Air Arabia does not hire you. (Note carefully Emirates' reaction to this question.) Check also about the specifics of the 'guaranteed' 1500 hours... earlier in this thread, more specific details emerged about this:
After successful completion of the MPL and an Air Arabia interview someone will only then be offered a line training contract. All who have successfully completed the MPL to date have been accepted for line training, but there is no guarantee of this in writing for anyone. The present contract is NOT a 1st Officer contract - what rubbish. It is a 750 hour written temporary contract with a verbal promise that all will get at least 1,500 hours. So at 750 hours AA will either extend for another 750 hours or offer a 1st Officer contract - it all depends on AAs manning requirements and perfomance during the first 750 hours. So your assertion that we will get an open ended permanent 1st Officer contract straight away from AA is plain wrong. AA is probably rightly assessing competence and will only wish to retain who it wishes to retain.
Whatever you are told by anyone regarding a training arrangement, has to be in writing. Well-organized, reputable companies will have no issues in doing this for you.
Good luck!