Well, it depends on how much content you need.
If you need icons or illustrations that cost less than $9
The minimum price of an icon is $2 and illustrations usually cost $5. If the total price of the artwork that you need is less than $9, then buying it one by one is the most cost-effective choice. For this, you can add the products to the shopping cart and pay in bulk or make a prepayment to your account balance and download the content in one click. Read about how to make a prepayment to your account balance.
If you need icons or illustrations that cost more than $9
In this other case, choosing to subscribe to either Pro Micro or Pro Standard plans might be better for you. With Pro Micro plan, you get a total of 10 credits for $9/month. Pro Standard subscription plan gives you 50 credits for $19/month.
Credits are Iconfinder's own currency and are used to download unique products. You will be able to see how many credits each product costs before downloading. Read more about credits.
The most important advantage of subscriptions is that all unused credits are rolled over for the following month, so you can always keep them for later. Read more about Roll-over credits. Learn about all the advantages of being a Pro subscriber.
If you need A LOT of icons or illustrations
Then, the Pro Ultimate subscription might be the best option. You will get 500 credits per month to use for downloading icons and illustrations.
Note: if you are not planning to use the subscription for longer than 1 month, you'll need to actively cancel it before the billing period is due, as you will be automatically billed for the following months, otherwise. You can cancel your subscription anytime from Your account page. Read here about How to cancel your subscription.