So no matter where you join at the end of the day all you need to do is work hard, be sincere and be true to yourself.
Let's jot down some few points.
Teachers- The teachers are qualified enough and they have ample amount of expertise in their own field. But they lack enthusiasm to teach.
Modules- This is a heavy setback for Aakash since they prepare extra detailed modules. Basically you won't be actually reading 3035% of its stuff because they are of much higher level and you won't be needing them for NEET.
Exams- Aakash conducts regular weekly tests and a once in a fortnight test known as AIATS.The questions are a bit above the level but it's standard enough to practice.
Doubt clearing sessions- The teachers clear your doubts whenever you need them to but I seriously sense the lack of enthusiasm and energy of the teachers as compared to the teachers of Allen.
Teachers- The best part about Allen are the teachers itself. They are more energized and prepared for your NEET than you are :p. They are highly qualified(IITians, Doctors,etc.) and they teach you really very well even though it's a class consisting of 200 students!!(Yes, you heard it right!)
Modules- Its just the opposite I have told about Aakash. The modules of Allen are the best pro points among all its other features. Simplified, Easy to learn, Illustrated, Very logical questions in the exercise parts.
Exams- Unlike Aakash, Allen doesn't have any weekly tests but it surely conducts those fortnight tests.The tests are sometimes way above the level but most of the timesthey give you a very brief insight about the questions that you will be facing in NEET 2018.
Doubt clearing sessions- The doubt clearing sessions are very good and you literally have a bunch of teachers just for a particular topic at any time of the day. They will be cooperative with your question solving skills and thus clear your doubts accordingly.
Some extra points
1.Allen gives you many revision books, last minute study handbooks, question booklets when the exam nears so that you have the best of the best preparations.
2.The whole faculty and the head of the institute will motivate you throughout the year and give you a positive vibe for the entrance test.
3.The teachers are specially instructed to eliminate your exam fears and to help out for the situations you will be facing in the actual exam hall.
So I just can only present all positive and negative points in front of you honesty. Decision should be yours according to your needs.
Hope it helps,