Hello @Shivam Ramola and welcome to Microsoft Q&A.
While both HDInsight and Synapse can run Spark, they are very different products.
HDInsight is always on and requires a great deal of configuration. It is suitable for heavy compute where you have very detailed needs. The learning curve is very steep. Most of HDInsights features are Apache based. There are several cluster types to choose from depending upon your need. HDInsight has been around for a number of years.
Synapse can be 'paused' , is consumption-based, and has a much more gentle learning curve. Synapse incorporates many other Azure services and is becoming a one-stop hub for Analytics and Data Orchestration. New features are being added faster than I can update this conversation.
If all your intended users are very technically competent, HDInsight is worth considering. If some of your users are not as technically inclined, Synapse may appear more appealing.
There is a new (preview) feature which allows some Synapse-HDInsight integrations. share-hive-metastore-with-synapse