We had an experience with both. The two systems are not really
comparable. LLVM is *huge*, and much more difficult to get into your
project. But since it's a generic framework for compilers, you get
lots of advantages -- the most obvious is that you don't need to worry
as much about optimizations. OTOH, Lightning is a simple native-code
generator, which is much easier to use, but it doesn't do more than
just that. (BTW, IIRC GCing code was not a problem in LLVM.)
(Since it's off topic, feel free to mail me off list for more
details, if you're interested.)