Hello Dbklee,
it is not possible to answer to your question as optical simply means they give you a fiber connection it can be a 10 Mbps bidirectional, more or less.
ADSL is asymmetric in nature with different speeds for download and upload directions but it is cheaper.
So I would say the difference can be this one.
>> can i do a vpn tunnelling (backup) to the adsl' vpn site-to site tunnelling + as an alternative path for the user to go to the internet?
if you want to have the user go to the internet using ADSL as secondary path this is possible it is just enough to NAT when destination address is not a private address of the other site.
Sending traffic over a vpn tunnel to site2 and then to go out to the internet from site2: this may be possible but it is not reasonable at least in standard scenarios.
Just to say you can exit to the internet and you can have a VPN connection exiting from the same link at the same time.
Hope to help
Giuseppe