I, too, have used iPod Touches (in tandem with a dumb phone) over the past several years. I haven't purchased a smartphone yet simply because I don't want to pay for the extra data plan. I am within reach of wifi 99.99% of my day and don't feel the need for the added expense. I am now seriously looking at the Nexus 4, and like what I see. I am also a Verizon customer, mainly because ATT's coverage was very poor where I live when I purchased my first cell phone many years ago. Since then ATT's coverage has improved, and the Nexus 4 might just be the reason I jump ship. You might check coverage in your area for sure. Go to
AT&T Cell Phones, U-verse, Digital TV, DSL Internet, and Phone Service and type 'coverage map' in the search box and see what you find out.
I think your question is a valid one. Do you purchase a new iPod Touch or a Nexus 4 and use it as a small tablet. As far as specs go, the only thing the iPod has going for it is storage capacity, 32 and 64 GB versus N4's 8 and 16 GB. Which may not be a big deal if you use cloud storage. Otherwise the N4 has it beat hands down, processor speed, camera, screen res, etc. The N4 even has a GPS receiver.
Another factor may be the amount you have invested in apps. But you are always going to be buying new ones and abandoning old ones. Over time, this may be a smaller issue than at first glance.
I've loved having a tablet-in-my-pocket over the last couple of years, but always felt more than a little handcuffed by Apple's app and store policies. Maybe I should get a N4 and port my number to Google Voice.