The tif only displays the first page, whereas the pdf opens in another viewer.
In my experience, very few TIF viewers understand and display multi-page TIF's. Instead, and as you say, they display only the first page. RM is no exception in handling only the first page of TIF files. This makes multi-page TIF's a non-starter as far as I'm concerned.
I view RM opening files in a different viewer as an advantage not a disadvantage. RM's viewer is not very good. For example, it renders census images in such a grainy fashion that they are essentially illegible. In RM7, the RM viewer runs in a modal window and therefore locks out all the rest of RM while you are viewing an image. This particular problem may be solved in RM8. I'm hoping so. If you "ok" out of the viewer instead of cancelling out of it, RM saves your file instead of just exiting. After a few save cycles, your JPG files become unreadable because JPG is a lossy format. Hence I link nearly all images into RM as files rather than as images so they will open in a viewer outside of RM.
PDF is sort of a special case. I have linked quite a few PDF's into RM, but I'm converting each one to JPG or PNG (sometimes to multiple JPG's or PNG's for multi-page PDF's) so that they will display on my Web site in a very natural way for a user. Browsers do much better with PDF files than they used to do, but I still prefer the way browsers handle JPG's and PNG's to the way they handle PDF's.