The most basic difference between API 620 and API 650 standards are

• API 620 allows the storage of liquids of low temperatures but high pressure. It can contain higher internal  pressure such as LNG. This usually applies to tank size larger than 300 ft.
• API 650 allows the storage of liquids of high temperatures but low pressure. This can be applied to any tank sizes.

Discuss with your project team to configure the requirements and you can choose a suitable standard for your design.

Also check out:

Difference between API 650, 620, 12D, and 12F.

Difference between API 2W 50 and API 2W 50T.