There is no best answer really, completely depends on what you want and your circumstances.
If you want a full experience of working with a designer and getting multiple different design options then no reason not to go for it. If your budget and timing allows (if your deadline for publishing isn’t less than 2 weeks).
Premade covers make great sense when:
- You don’t have a budget for completely custom cover and it’s under $200. There is a ton of awesome premades available for below that price.
- You don’t have much time left before your deadline for publishing. If for some reason you have a week or less to get a book cover, quickly browsing premades and buying something that matches your book will be a lot faster and cheaper than hiring a designer to design covers extra fast. Extra fast means extra pay usually.
- You’ve not written the book yet but saw a premade cover that gives you inspiration and idea to write the book in future. Quite a few authors buy them early in that case, some even hoard them. If the cover inspires an idea and helps write it, it might make a lot of sense to get it.