In your case I think you don´t need Merge two different trees, because your trunk and branch have a relationship, so this not what you need.
Reintegrate a branch is that what you need if you want bring changes from branch to trunk.
You can use Merge a range of revisions for merge from trunk to branch (update to newest stable version), but also for merge branch to trunk (I work mostly with this option).
Maybe this SO-Question can also help you and here is the “standard literature” for the topic.
Update:
(Tortoise) Merge-Options:
Update II:
(Tortoise) Merge-Options in current Version:
“Merge a range of revisions” is normally used for a “branch to trunk”-Merge.