Well, the simple anmd most obvious answer is to shoot in whatever format you intend to edit and deliver in. If you intend to delilver a widescreen presentation then shoot and edit in widescreen.
Frankly, I've never tried shotoing 4:3 and tried to edit it into a widescreen movie. Doesn't make much sense to try if you can shoot in 16:9. Try it and see if you like the results but it will lower the quallity as you will have to zoom the image to fit wither way around.
But to your other question. If you trv19 is anything like my trv33 then it is shooting in true widescreen. The only reason you have black bars top and bottom of the camera screen is that the camera screen is not 16:9 and the camera is 'shrinking' the image slightly to fit. When you capture you should find you have an anamorphic (alomst) picture. i.e. short fat people (just like me

) who are just waiting to be stretched horizontally for display on a widescreen TV or in your editing s/w. If in premiere (say) you start witha widescreen preset then the preview monitor will do the stretching for you during edit.