I would also recommend T-Mobile if you have adequate coverage. However, I would also like to share some information if you are more interested in AT&T.
AT&T is offering LTE on its prepaid service, GoPhone. Since the Nexus 4 can't use LTE on AT&T, this would be pertinent to people who wanted to use the HTC One or Samsung Galaxy S4 Google Editions. The official rollout of LTE for GoPhone won't start until June 20th, but you can dial 611 and ask customer service to provision your SIM card manually for LTE right now.
One of the GoPhone plans offers unlimited talk, unlimited messaging, and 1 GB of data for $65. (There are rumors that say you can add 1 GB of data for $10/GB up to a maximum of 3 GB starting on June 20th. The current price is $25/GB.)
AT&T can support the Nexus 4 on its HSPA+ network, but it might be better to use an MVNO. Airvoice Wireless offers unlimited voice, unlimited messaging, and 1 GB of data for $55/month. Red Pocket offers the same but bumps up the data to 2 GB for $59.99/month. Both MVNO's run off of the AT&T network, but at a cheaper price. (There's no word yet if they'll get access to LTE, though.)
So there seems to be cheaper options with 1, 2, or 3 GB of data using AT&T's network, but if you want unlimited data, you'll need to stick with a grandfathered unlimited plan or switch to T-Mobile. It's best to first find out how much data you use in a month on average, then select a plan that best fits your needs.
In terms of value, the difference between an unlimited plan on AT&T and their 1 GB GoPhone plan is $50 ($115 - $65) per month. Over one year, that is $600, or the price of one brand new HTC One Google Edition. In other words, you could upgrade your phone every year with the savings.
Over two years (which is the typical length of a phone contract), that ends up to be $1200, or the price of one Samsung Galaxy S4 Google Edition, one Nexus 4 (16 GB), and one Nexus 7 (16 GB) combined. Going this route, you could upgrade every 2 years with 2 new phones and 1 new tablet. Of course, you will have to decide for yourself what is best in terms of value.
I am thinking about buying a off-contract phone like the Nexus 4 or the recently announced off-contract GS4/HTC One. I was wondering what is the best value in terms of carrier service. Currently I have AT&T with unlimited data (grandfathered in) and I pay around $115/ month. Since I wont have to sign a contract is there a better value carrier plan I should look into?
Note: I'm not looking for the cheapest carrier plan just the one with the most value. Though the goal is to reduce my monthly payments without sacrificing too much.
I have a feeling that if I want to keep my unlimited data plan that I will have to stick with my current plan, but none the less please let me know of any options I may have for a more valuable carrier plan.
Thanks in advance.