You need to know that an email consists of the envelope and the message itself. We admins care only about the envelope and the users about the message, because we are just caring for the transportation. SPF and SRS both work on the envelope layer.
If you have higher volumes and the administration of the forwarding addresses will be administered by the admin, it will be easier to set up SRS and use aliases. Thus, the mails will be processed by the MTA only.
If the users should be able to adapt the forwarding process, a sieve script could be easier, but this an additional component (MDA plus sieve itself), that could fail in some way.
Spam detection and filtering should happen in both cases before the email is further processed, and is therefore no problem. Make sure you drop the spam mails instead of only marking them as spam.