Don't take this as any legit comparison, but from my perspective: Fuel is at 1.0 RC1 as of right now, although it has only been started a few months ago. Lithium has been in development for ages (since 2009) and it seems like it hasn't seen a lot of action lately, the last blog post on http://lithify.me/ being from November 2010 and the last packaged download being 0.9.9 from December 2010. (Didn't check the activity on the repos.)

That doesn't tell you a lot about the quality or reliability of the framework and what's to come but personally I prefer the "let's get this out there" approach over borderline vapourware.

Besides that, browsing the FuelPHP forum it looks like people are already busy adding functionality through plugins and the team seems to be helpful and embracing it. So they've definitely got 'momentum'.

Also the 'docs' on FuelPHP seem to be pretty 'complete' at first glance, also giving me the impression that they are totally up-to-date while I can't even figure out where the proper 'docs' for Lithium are. They don't seem to have things arranged for 'being in the spotlight' yet.

And I can't comment on what's going on under the hood really, as I am more interested in 'libraries' that are not depending on any framework these days.

But as far as I can see, FuelPHP is sitting there for you along with current docs and a dedicated team to try out and give it a shot.