[Exim] Efficiency of Mail Scanning

I am shortly to test an external spam-scoring program.
In the aye-aye book and on the list, there are several examples on how to set
this up, either system filter, director/router transport or user filter.

Quote:
"An alternative way of arranging for messages to be passed to a scanner
program if to make use of a system filter ...
"However, in several ways this is more complicated"

This implies that a system filter, although possible, is less desirable. No
benefits of such a setup are given.

My question is, therefore, this:
All incoming mail is to be scanned. This includes incoming mail for users,
and incoming mail for submission to majordomo. Obviously it is more efficient
for a mail to be scanned once, before distribution to several recipients. But
which is going to be technically the most resources-efficient, system filter
or a conditional director/router ?