"What are the trade-offs, then, between Portra 100 and Portra 400--from both an aesthetic as well as a technical standpoint?"
Mainly the obvious shutter speed/aperture and granularity. However, the reality is that we're starting to pick the fly poop out of the pepper. You simply can't go wrong. The Portras are nice true negative films that print in a straightforward manner. Ektar has a "juiced up" color look that many find truer to life (closer to Reala or Kodacolor 100 but with finer grain), but getting the balance right can be challenging under some lighting. If you are new to printing color optically, I suggest Portra 160. Fine grain and pleasing colors without the (for me anyway) challenge of getting the cyan (print color, not enlarger setting) just right with Ektar.