The people should decide.
Also, I don't think a merger with Switzerland would make a whole lot of sense politically, culturally and linguistically speaking; Austria seems much more suitable in this respect. If there's an Austrian region that should join Switzerland it's Vorarlberg.
If the people give the SVP another absolute majority today, it's kinda like "the people decided". For the status quo.
Merging Südtirol with Switzerland would look pretty weird. Südtirol would be some sort of
Schweizer Blinddarm (Swiss vermiform appendix) ...