I pitch jigs to any cover I can get them through....trees, weeds, cane, whatever else.
Once the surface is matted or the weeds are a bit more choking I go to a small t-rigged craw.
So I guess my rule of thumb is I fish the jig until I'm annoyed with getting it snagged up or it not getting through the mat. I then turn to the T-rig.
Weight to me is not as much an issue b/c I have different weighted sinkers for my t-rig and jigs.