What platform will users use?
A key distinction is to consider the platform your users will be using primarily. Mouseover tooltips may work fine for desktop/laptop applications - but smartphone/tablet users may never see these.
This is perhaps part of the reason sites like Twitter or other "often navigated on mobile" sites use tours. A tour is considerably more friendly for a mobile device. It also explains why they often have "panels" you can switch between. On a computer this seems a bit awkward, but on a mobile device, being able to swipe between the pages is very natural.
Who are they and what will they do with the tool?
Who are your users? Are they "Internet savvy" people who will skip a tour entirely and just figure things out themselves? If so, just skip the tour. Is it older people or otherwise disconnected people? Then perhaps the tour is useful.
Also consider, are you designing a site with considerable interactions? If so, perhaps tooltips are useful. Is the site designed to be a consumption source? A tour may guide users into where/how to find information much more easily than tooltips.
The point of this is: figure out what your user needs are. Then design the solution to meet those needs. You can't pick "which is better" without a clear understanding of the types of tasks your users will be using (as well as platform).The better UX may be clear, understandable tooltips -- depending on your use cases. Or it might be a tour.
A note on tooltips
Regarding tooltips, it is absolutely critical when using tooltips that they are user decipherable, consistent, and not hidden. For example, consider on Stack Exchange:
- Mousing over a tag is actually a tooltip, presenting the tag description
- Mousing over up/downvote buttons show tooltips
- Mousing over the star shows it's a favorite tag
- Mousing over the "edited 2 hours ago" link shows the exact time that action occurred
- Mousing over the sort options on this page shows what the criteria means
How many of these were you aware of? I'm sure there are tons of others too. I have been using Stack Exchange for years and still find new ones (go to chat if you want to go on a hunt for obscure tooltips).
Tooltips are relatively natural and expected by users, this research (Heidmann, 2003) discusses map navigation, emphasis mine:
The results of our two usability tests show, that the tooltips itself are a very easy-to-use interaction style that was
immediately adopted by the users. They could retrieve object labels without leaving the small-scaled overview. The
results also indicate that hidden labels for map objects are sufficient for some tasks. In the usability tests, some tasks
required to get a general overview of a spatial configuration, like identifying problematic, unfavourable parts of a
route, or learning the route in order to sketch it afterwards. Most subjects used tooltips to complete these tasks, even
when other possibilities were available. It seems that users preferred not to leave the general view and to read additional
information in the tooltips to build up a mental model of the spatial and temporal structure of the tour.
Note that in map navigation there is a user expectation to be able to mouseover and see a tooltip. This expectation may not exist in other applications.
Again, the key is identifying your user tasks and designing based on those. Do not just design a great site and expect users to figure it out. Identify their tasks, needs, and background - then design your interactions and functionality.