Hello Vikas,
The difference is big, but one of the most important things that many times people forget is the product lifecycle and compatibility.
As you said, there are ongoing update releases for both products, but their lifecycle is completely different, and the releases are going to stop someday for v8.2 and for v9 they will continue. So keep this in mind. If you are going to upgrade your environment, update it to v9 and forget 8.2. Pay attention also to your environment. Check compatibility lists to check if your environment is supported, for example:
docs.microsoft.com/.../software-requirements-for-microsoft-dynamics-365-server
Regarding the upgrade, you can do an upgrade directly from 8.1 to a v9 environment, importing the DB in Deployment Manager. You may have some sort of error, but that must be with a support case. And that may happen with v8.1 or 8.2, it doesn't matter. But hopefully, all runs fine.
Hope it helps.
Best regards
