Well, the nexus 5 has a slightly shorter battery life and worse camera, but is significantly cheaper. However, the nexus also comes with 'stock' android 4.4, and awesomely fast updates. The G2 also has buttons on he back, a turnoff for some people. Really, they're both good phones, but as a nexus 4 owner, I'd suggest the nexus 5. (I like my n4.)
Both have the same performance, both lack a expansion sd card. G2 has a slightly larger screen.