The main differences are:
1. Base version has a restriction on the # of interfaces. You can setup inside, outside and one DMZ. Additionally you have to restrict communication between the inside and the DMZ.
2. The Plus version does not have this interface restriction.
3. Base version has limitations on the number of VLAN's supported.
4. Base version has also a limitation on the numbers of VPN supported.
Regards,
Carlos Roque