They are all based on the same TomatoUSB code. But each developer has his own way to do things and to find out exactly what he has done, you have to read up in the forum posts, readme files (usually on their download site) and best of all - look at the source code listing on the repo.or.cz/tomato code repository. There, you will see releases from developers carry a "tag" with the version number, and by clicking on that tag, and then "graphiclog" you can see what is in each build.
1) Has Transmission bit torrent client, it's biggest feature.
2) Has Toastman features, nothing huge, mostly extra facilities for QOS. realtime and 24 hour client monitoring and statistics. BUT - the EXT (Extras) builds also include a lot of utilities which some people may want to use, that you must enable and handle by yourself, usually from the command line or scripting. That's why it's bigger.
3) Has Victek's Web Portal built into it.
You can see what is in my builds, on the 4shared download site - the readme files in the build directories, and also follow the releases on Linksysinfo.org tomato forum, which is currently more active than this one.
If you are not using the portal, the torrent client, nor anything complex, flash the smallest builds that have the features you need in them. STD or MINI (there's sometimes even a "tiny" build available. There's no point in using the EXT version on your router.
Most features don't actually run if you do not enable them. But they do make the build larger and thus if you don't need them, and you are short on space, don't use the bigger builds. However, as time passes your router will slowly miss out by not being able to run the latest mods, because of slower processor, fewer resources.
Standard TomatoUSB/RT has not had an official release for a very long time now, but Teddy Bear's code has instead been posted into the repository and has been used by the other modders. He recently posted a lot of new updates, as far as I know I am the only one to have incorporated all of them into releases so far (but I may be wrong about that).
I hope that's useful.
The fastest and easiest way to see what is in a build is to flash it and see. It's actually quicker than trawling through the data on the forums.