Should I put Scope:Link or scope:Global IPv6 address in DNS AAAA record?

Yes, precisely so. The globally-scoped address is the one you advertise to the world (which in this case, means list in your AAAA record). The link-scoped address is a very useful feature of ipv6 which makes setting up point-to-point links elegant, but it isn't, as you say, globally-routable, and advertising it to the world will not get you any visitors.

As for SPF, you would certainly want to list the globally-scoped address. I can't offhand think of any circumstances under which you would preferentially make connections to another local mail server from your link-scoped address, so I see no point in listing it, but others might know better than I (I'm talking to you, @SanderStefann).