The way the new and old income tax regimes handle deductions and exemptions is the primary distinction between them.
Standard deductions under both regimes are the same. It means you can avail benefits of Rs 50,000.
The deductions and exemptions are important in lowering your taxes. Under the Old regime, you can avail of benefits of approx 120 exemptions. On the other hand, the new one offers few concessions.
The calculation of taxes under both regimes is different due to the difference between their tax slabs. For instance, income up to Rs 3 lakh attracts zero tax under new tax regime while it is Rs 2.5 lakh under the previous one.
If you fail to opt for the tax regime, the new income tax regime will be the default option. So, inform your employer about the same.