In my opinion downgrading your SL to Leopard box instead of upgrading your Tiger box to Leopard is a long term mistake.
Lion is coming this summer and you're only putting off the inevitable.
i've seen similar issues when trying to share folders from an older mac OS
always a good idea to share folders on the mac with the most recent OS and then connect from the mac with the older OS
i have found it a good rule to follow
newer OS probably is backward compatible while older OS isn't...