Portra is a low-contrast, low-saturation film made primarily for wedding and portrait photographers (it's the successor to Vericolor III Professional). It's designed to capture the details in both the white wedding dress and the black tuxedo at the same time, while rendering pleasing, blemish-minimized skin tones. There's nothing special about its grain structure or resolution; it's about par for the course in last-generation colour films. As with most colour negative film, you can increase the saturation and contrast by slightly overexposing (shooting the 160, for instance, as if it were ISO 100) without any special development (as long as you don't go over by more than 2/3 of a stop), but that just gets you into the same sort of range as general-purpose consumer films. Pushing it (underexposing by 1 or 2 stops, then having the film developed to compensate) will result in a grainy pastel-coloured image. This film would definitely not be your first choice for shooting autumn foliage, but if you want to flatter people or create something just a little bit on the sentimental side, it's a great choice.
Superia is a general-purpose film designed to be a bit "snappy". The colours aren't super-saturated, but they're definitely there in an unmistakable way, and there's a lot more contrast in the midtones. (You can bring the saturation and contrast down a touch by very slightly underexposing; again by no more than 2/3 of a stop, and you'll suffer a slight grain penalty doing so.) You'll want to be a lot more careful shooting people; if you want a polished, commercial look for fashion-type shots, then good makeup isn't optional. As with most films designed for slightly higher contrast and saturation, the grain is a little bit smaller and "tighter". If you want "candy colours" in a fashion shoot or are shooting things other than people, this is probably the film you want (if Portra is the alternative).