Subscription vs. One Time Payment

It depends --- on the nature of the product.

Does a subscription offer a real, significant, tangible benefit or advantage to the user/customer?

Subscriptions almost always cost more over time so this needs to be clearly justifiable to the customer.

A subscription offered mainly to benefit the developer/publisher is a marketplace failure waiting to happen IMO. Its only a matter of time before the customer puts 2 and 2 together and switches to a standalone product that costs less.

This a SaaS cookie cutter solution portfolio thing? If so, perhaps subscription.

If not, then it sounds like a regular design studio or IT solution vendor - so either time & materials or fixed fee contracts are normal.