In limewire you are more prone to ads, and some will come up with explicit things. Frostwire is great for me, haven't had a virus. Don't download .mp3 files that are shown as folders or .exe, etc.
I've gotten a couple of viruses with limewire, but non with frostwire.
I think its because more people use limewire, meaning more people will try to hack your PC.