I have minimal experience with libevent and much more with libev, but I do think that libev is very, very well-engineered. The author is on steroids, you can really notice it just by reading the documentation. The documentation and the code indicate that he knows what he's doing and is very serious about performance. There are also no obvious design flaws in libev that would result in limitations. Want fork? No problem, but be aware of clearly documented issues A B and C. Want threads? No problems, it even documents how, as well as potential issues. Want signals? No problem, and again potential issues are clearly documented.
Niels (of libevent) is also pretty ridiculously talented; it's also hard to dispute that libevent has had more testing.
Libevent vs. libev really seems like a Linux vs. BSD kind of debate. I'd always choose libevent over libev, but you can apparently get a performance improvement (which is probably going to be marginal compared to other simple things you can do to speed up an evented program) by going with libev. And it looks like if you're doing clientside dev, like building a new browser or file transfer client, that libev is easier to embed.
