Lots of good advice here, so I'll just add two quick things:

Pay attention to the amount of force required to grip the shifter in order to twist. I've worked with some customers with arthritis, and the ability to operate a twist shifter seems to vary greatly, depending on how their hards are affected.

Not all indexed left/front twist shifters have trim, and it makes them a nightmare to ride with or wrench on. The nice ones, IME, are "micro-indexed" and almost infinitely trim-able. Cheap twist shifters seem to be worse for setting up than cheap thumb shifters.

My 2 cents, of course.