Encrypted data needs to be stored as varbinary. You cannot also store the decrypted value.
However, if you also store the decrypted value, there's arguably no reason for you to encrypt the data.
If you wish to decrypt for calculations, you can do that on the fly in queries, however be aware that encryption/decryption uses lots of CPU.
I assume you mean you will use a symmetric key for encryption/decryption and protect that with a certificate. That's what I'd recommend. Note that your users will need VIEW DEFINITION on the symmetric key and CONTROL on the certificate.