Are you looking at the AT&T Optimus G (LG E970) or the LG Optimus G (LG E973)? I don't know if the E970 can be unlocked, and I don't know if there is a US version of the E973.
Off hand, though, I'd say get the Nexus 4 from the Google Play store. It will be much less expensive, it will most likely get better support and updates, and if your choices are the Optimus and the Nexus, the Nexus will certainly meet your needs. Note that you will be getting the 'Pure Android Experience'. It's your call as to whether that's a plus, a minus, or a null. Like the E973, and unlike the E970, it doesn't have LTE or a MicroSD slot. The other drawback it that it only has 8 or 16GB of memory. This is plenty for a typical user, and as many have pointed out, non-typical users always have access to the cloud.
If I had to choose between these two, I'd definitely get the Nexus. Still, it does matter which version of the Optimus you're looking at, because they do have different features, profiles, and limitations. Check PhoneArena for a good comparison: Google Nexus 4 vs LG Optimus G vs LG Optimus G AT&T - Phone specs comparison
Finally, according to The Verge, "Compared to the LG Optimus G, which shares many of the same components, it's no contest — the Nexus 4 is a far nicer piece of hardware." (The Nexus 4: Google's flagship phone lands November 13th for $299 | The Verge
EDIT: I'm not sure how technical you want to get, but as far as HSPA+ vs LTE, HSPA+ is not, technically, a 4G technology. If I understand it right, HSPA+ 42 is as fast as the technology can go, whereas LTE has a lot of headroom to grow. Also, from what I understand, T-Mobile will be adding LTE service to their network. IIRC, they were saying something like 2013/2014 for the rollout. That's just information I gleaned while trying to pick out a phone for myself. I wasn't looking into this specifically, so a lot of that info may be off.