I think a better design is to use both links as much as possible.
The main reason is redundancy, or load-balancing, or both.
Then use QoS for traffic prioritization. But this setup is not really
as simple as it looks.
As for the access-list configs, Extended ACL should be used.
A sample config could be like:
interface Ethernet0
ip address 172.16.1.1 255.255.0.0
ip access-group 100 in
access-list 100 permit tcp any host 172.17.1.100 eq smtp
access-list 100 deny tcp any any
access-list 100 permit ip any any
*The above config simply means, allow ANY source host's packet coming IN
Ethernet0's interface to be forwarded to the Mail server (172.17.1.00)
using only SMTP port (port 25).
Check the following link for info about ACLs:
http://www.cisco.com/en/US/products/sw/secursw/ps1018/products_tech_note09186a00800a5b9a.shtml
Goodluck.