Should I buy subscription or perpetual license?

It depends on your usage scenario. 

For the latest information on pricing plans, licensing, maintenance and subscription options please visit:

https://www.spectralcore.com/fullconvert/signup

Currently we offer two pricing plans - Standard and Pro. Plans differ in the number of supported databases and the ability to synchronize data differences (Pro).

When is it better to buy a perpetual license?

In this case, perpetual license does not expire - once you have it, you can use the app indefinitely. Maintenance subscription associated with perpetual license is billed yearly. 
Maintenance subscription can be canceled anytime.
While the maintenance subscription is active you can get the latest version of the app and the support via email or online meeting.

If you are only going to use a single application and not use it very regularly - and it's quick and easy for you to extend the maintenance subscription.

When is it better to buy a subscription license?

With license subscription, you are certain you have access to download of the latest version and access to our technical support. You don't have to worry about maintenance subscription - it's included, as charges are recurring and you know you're all set until you decide to cancel. License subscription can be canceled anytime.

Also, if your purchasing process is long and complex, it makes sense to purchase a license subscription instead of going through the same process when you need to upgrade the application.