Size comparison between a Composite type and an equivalent Text field

I need to store very large integers (more of 30 digits).

I found two solutions to this problem:
- using a text field
- splitting the integer into 2 parts and then storing them in a
composite type with 2 bigint fields

The definitive choice will depend on the disk space used by one solution
instead of the other.

I think the storage size of the text field will be the number of the
digits plus some extra data for the structure of the text type.
For the composite type the size will be of 2*8 bytes plus the extra data
for maintaning the structure of the type.

For example if i have an integer of 30 digits:

Text Field
30 bytes + sizeof(text data structure)

Composite type
8 bytes + sizeof(composite data structure)

What of the two data structures (text or composite) will use more disk
space?

Thank you,

--
Doct. Eng. Denis Gasparin
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Edistar srl