tl;dr Personal Preference, both are good, get what suits your needs best.
Coming from someone who owns both devices, I think you should make your decision based on your needs. Are you alright with the Optimus V right now? What would you want to get out of an N7 if you were to go that route? You said school is starting up soon, is that going to factor into your choice?
The Nexus 7 is fantastic for web browsing and media consumption. It is very fast and for $200-$250, no tablet compares to it. It is Wifi only though, so if you plan on using it at school or on-the-go, you might have some difficulties. It is blazing fast though, and Jelly Bean is absolutely awesome in terms of speed and fluidity. I can't get enough of the pure Google experience.
Coming from an Intercept and Triumph on VM, the Evo V is a HUGE upgrade. Everything is much smoother, and simple,everyday tasks can be performed very easily. I can't speak on the Optimus V, but the Triumph and Intercept both had many glaring issues that prevented them from performing simple tasks. For example, the Triumph GPS was garbage, and IF you got a location lock, it was often very inaccurate. The Evo has no big issues except the texting errors that I believe have to do with coverage, so if you have good coverage you should be fine. 4G is spotty and you said you have none, so don't expect to have it anywhere else either.
Overall, I think it really comes down to what fits your needs best. Both the Evo V and Nexus 7 are very nice devices. If your Optimus V is having problems performing tasks you use it for everyday, an upgrade to the Evo would be very usefeul and welcomed for you. If you are fine with what your phone is giving you, a tablet would also be a nice investment. Heck, if you want a little pure google and a phone upgrade in one package, the Galaxy Nexus also wouldn't be bad option like others mentioned above.