This is what I found regarding this data type on msdn
ntext
Variable-length Unicode data with a maximum length of 2^30 - 1 (1,073,741,823) characters. Storage size, in bytes, is two times the number of characters entered. The ISO synonym for ntext is national text.
text
Variable-length non-Unicode data in the code page of the server and with a maximum length of 2^31-1 (2,147,483,647) characters. When the server code page uses double-byte characters, the storage is still 2,147,483,647 bytes. Depending on the character string, the storage size may be less than 2,147,483,647 bytes.
For more you can check following threads:
ntext, text, and image[
^]
char and varchar[
^]
Hope it helps.
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