USB-OTG vs portable Sdcard I have been in the market for an Android tablet. I have found a few within most of my desired specs. Specifically the Nexus 7. This device lacks expandable storage, as do other similarly priced devices  My intent is to root the device and upgrade to 6.0.1. I prefer external storage for various ROMs and backups where they are safe from accidental wipe. My question would be is a USB-OTG system a comparable alternative?
First thing is to verify if the device you propose to buy has USB OTG support. Apart from verifying from OEM specs, do see this How can I determine if my device has USB Host Mode (OTG) support? and also the listing  of supported devices by Chainfire. Saw your edit about Nexus 7. This post confirms OTG support on Marshmallow
For your stated purpose of storing ROMs and backups it should work well. If you are using TWRP recovery , this thread may help, TWRP Using USB OTG to backup? 


I have seen my friends  use it thus but haven't used it personally- all my devices had external SD to store