In addition to the question of what the invigilators will allow you to use in the exams, it is also worth considering the following.
1. The vast majority of the (probably thousands) of ATPL students that I have met have used the CASIO Fx83 or the CASIO FX85. So if you get one of these and you are doing a full time course you will be surrounded by other people using the same calculator. This helps if you are not fully up to speed on how to use it. The instructors will also be fully familiar with it.
2. When you get into the exam room there is a remote chance that the invigilator will decide that he or she does not like your calculator. If you are using an FX83 or FX85 you can say "well if you are going to take mine away from me then you need to take them away from virtually every other student in this room". You will almost certainly get to keep your calculator.