Well, it depends on how many icons you need and how much they cost.
The minimum prepayment accepted is $10 and the minimum price of an icon is $2. This could allow you to get up to 5 icons for that prepayment. If you instead chose to subscribe to Pro Micro, you would get 10 downloads for $9.
So, if you need 5 icons or more, getting a subscription might be a better idea.
Advantages of Pay-as-you-go
The advantage of Pay-as-you-go is that it gives you freedom to buy icons whenever you need them, without worrying about monthly subscriptions that auto-renew. However, you will need to pay attention to the license agreement of each icon. Read about licenses in the License overview. Read here about how to make a prepayment to your account balance.
Advantages of subscriptions
Choosing a subscription is the more cost-effective option for getting icons. The advantage is that it is easy to use and you won't have to worry about frequent payments and about your account balance. Additionally, you will get all the icons under the Basic license (commercial use, no attribution), so you do not have to worry about the license agreements of the icons, as in the case of Pay-as-you-go. Read about the advantages of Pro subscriptions.
Check out this comparison between buying options.