Hello there,
If you are planning to attempt both the sessions in JEE Main, then the April session is an advantage for you. In case you do not get a good rank in the January session, you can work on yourself and prepare for a better result in the April session. The best of two will be considered during your admission in the colleges. Thus, if you get a good rank in April then this rank will be considered during your admission.
Hope this answer helps you.
Good luck!!