The differences between 1.4.0 and 1.4.9 are mostly bugfixes and minor improvements, so your question really boils down to the benefits of going from 1.3 to 1.4. One caveat:
It is worth noting that WICKET-2846 will almost definitely be reverted in the next release because the community has shown a preference for this. So, it would be wise not to take advantage of the InheritableThreadLocal that was put into version 1.4.9.
-Wicket main page
Generics, which you mentioned, certainly are the most visible change, and the one that will likely involve the most work if you are annoyed by warnings. See the Wicket wiki on upgrading and the expanded news post for a more comprehensive list of the changes.
My team at work migrated a few months ago and didn't notice any big effects, good or bad. It did take a few hours to update the generics references, but there's some comfort in knowing you're not using past-end-of-life technology, too. We don't use Spring here, but 1.4 is supposedly much more compatible with Spring than previous versions were.
In your case, it may just come down to what you find more annoying: spending a few hours generifying your code or using something that's in end-of-life status.