I'd discourage Portal creation other than in Dev orgs - any new Portal developmnent should stand on the shoulders of Communities instead. If you replace
Community, a lot of the existing documentation holds true.
Also watch out, I don't think you will get a Portal or a Community to live on your Salesforce domain.
Which one of these portals would be the best for what I'm trying to do?
If you must use a Portal, go with a Customer Portal. You will need self-registration functionality which I don't think is supported by Partner Portal.
How long is salesforce going to support said portal? (I saw some talk of self-service being outdated)
There are no plans to cease support of Salesforce portals. Existing portal customers will have Communities functionality provisioned at no additional cost
Once I get the portal created what's the best way to link it to salesforce.com?
Playing around with the Login Settings on a custom subdomain in a Developer Edition org. Try out the
High Volume Customer Portal profile and create
Sharing Sets to expose your Salesforce data to visitors based on linkages of records to their associated user's Contact or Account.
Note that the going rate for Customer Portal (Community) licenses are about US$5.00 per user per month. The going rate for Partner Portal (Community) licenses are about US$35.00 per user per month.
You may be expected to purchase a minimum headcount (perhaps 100 for Customer Portal, 20 for Partner Portal) at a minimum subscription period (perhaps 12 months).