With UNICODE UTF-8 the basic (a-z, A-Z, 0-9, ...) 128 characters
(there are actually less than 128) are single byte characters
identical to the original ASCII specification. All other characters
might have multiple bytes.
This means that as long you are transferring roman alphabet based
text, the impact will be very low since the text will mostly consist
of those 128 characters.
for other languages more characters consisting of multiple bytes
would be transferred.
I don't know about PostgreSQL's internal treatement of multi-byte
characters and whether this woud require more CPU time.
After weighing pro and cons, I'd definitely go with UNICODE.
Marc