See honestly you can choose only one exam to give out of these two exams. You can try to give ISI on May 6 by booking a slot. If it's not possible then you can give NEET as it's the most important entrance exam. Now you have to give any one exam and the decision is purely based on your first preference meanwhile try to see the time of both the exam do not clash on 5th may then there is chance otherwise decide for which exam you want to sit.
If you have not registered for the isi then check if there is any other slot that is available for giving the exam.
All the best!