Creating a bunch of variants manually allows for more granular control and personalization, as you can customize each email according to the recipient's industry, role, interests, etc. However, it is more time-consuming than using spintax.
In practice, the best approach is often a combination of both. You might use spintax for parts of the email that can be relatively generic while manually customizing other parts to better fit each recipient.
The best method to use depends on your specific use case:
Spintax can be great for simple alternations and is compact, which is useful when space is a consideration. However, it can become complex and hard to manage for larger texts with many variations. It is also harder to ensure that all combinations make sense or sound natural.
Variants are easier to manage for larger texts, and you have full control over each version, so they can be more carefully tailored to sound natural. They can also be easier to use with advanced language generation tools, as the variants can be treated as individual examples. However, they take up more space and may require more effort to create.