Astia 100: pro version of Sensia, lower contrast (as E6 goes), natural colors, handles a variety of lighting well, nice skin tones.
Provia 100: high contrast, saturated colors, goes blue in the shade, finer grain than Astia.
Velvia 50: high contrast, very saturated warm colors, can look weird in some lighting, finer grain than Provia.
I would recommend starting out with Sensia, that way you can afford twice as much film.