Do you have unlimited data? If so, there is no other network currently that compares to Verizon in terms of the depth of 4G LTE coverage and no other network that will let you have truly unlimited, unthrottled data. Whether either of those are important to you depends on where you live, how much you travel and how much data you use. Once you give up unlimited data on Verizon, you can never get it back. ATT no longer offers it and Tmobile and Sprint are way behind in building their 4G LTE networks. They will catch up, but not in the next 2 years.
As for the Nexus 4 - it is HSPA+ speeds only. Whether that is important to you or not depends on the Verizon 4G LTE speeds that are available to you where you live/travel. Where I live, my 4G LTE speeds rarely exceed HSPA+ speeds. The unlocked 8GB Nexus 4 is only $299 from the playstore but you need to research what the postpaid and prepaid options are for your area given the ATT and TMobile network where you live. You could end up saving a lot on your monthly wireless bill if TMobile has decent coverage where you live and you go with Straight Talk prepaid or if Net10's 1.5GB data limit on the ATT network works for you.
As long as Verizon uses CDMA technology, you will NEVER get android upgrades through Verizon that compare to what you can get with an unlocked GSM Nexus directly from Google. And Verizon obstructs the upgrades even if you have a CDMA Nexus on Verizon like the GNex. The plan is to eliminate CDMA from the Verizon network, but idk how long that will take, but I can't imagine it will happen in the next 2 years. And even when it happens, Verizon QA testing may still be a bottleneck to an update.
But if your upgrade isn't until August, the Nexus 5 (or whatever it will be called) may be on the horizon by then.