Transport streams do not play on phones or in browsers and are awkward to edit. 3GP makes very small files that old phones can play. TS is at home in broadcast recordings and camcorder footage (MTS, M2TS); 3GP is the usual choice for video from older mobile phones.
The defaults suit most files. Under Options you can lower the resolution, trade quality against file size, remove the sound and trim the start and end. This is demanding work for a browser: a long or high-resolution file can take several minutes, and your device does all of it, so keep the tab open.
This pair uses the compatibility engine, ffmpeg built for the browser, because 3GP cannot be written by the browser's built-in codecs. It is a one-off download of about 10 MB that the browser keeps, it is slower than the built-in codecs, and it takes files up to 1 GB.