At first glance, IMHO, when comparing Tag&Rename to mp3tag the easy answer is Tag&Rename, but mp3tag has a deceptively simple interface. Lurking under the surface is a scripting language that is compatible with Foobar2000's Tagz scripting language. Users are free to write their own scripts for many things including tagging from web sources that don't come stock with the program. Actions may also be scripted that can perform any number of operations on your files with the scripting language being powerful enough that your imagination is the only limiting factor. You don't have to reinvent the wheel, either, as their forum is full of actions and scripts you can add to your installation easily.
What cinched me as a user is unlike most tagging programs, Tag&Rename included, you are not boxed into a pre-determined set of tags that the program author feels should be all you will need. You can use mp3tag to create, edit, and delete any tag. I'm talking any tag written by any tagging program ever & you are free to even create your own tags. Even if you have no interest in creating your own tags maybe you have some music files that were tagged by some other program you were using 5-10 years ago.
Who knows what weird tags are lurking in your files that your old program utilized that your new one doesn't? Well, mp3tag knows and gives you the ability to look at the raw list of tags & add, edit, or delete any your new tagging scheme & tagging program don't use. For a specific example, this is especially useful for those who used MusicMatch back at the height of its popularity. MusicMatch used a LOT of custom tags that no other tagging program uses. Loading up these files in mp3tag will allow you to nuke those relics from the past with impunity.