Trade-Offs

R-specific byte-compiler and byte-code VM should match R semantics very well

Byte-code VM is in R source code, so harder to change for individual users (and distribute to other users)

LLVM has more resources, people, etc. working on it and so may improve more rapidly

R is very portable. LLVM is also and targets different hardware such as GPUs and even output to VMs and languages such as JavaScript.