Under what circumstances would you use a ribbon tail worm vs. a straight tail worm?

Probably more info is needed on style of straight worm you are referring to. There is such a variety of "straight worms" including senko styles and even then, length comes into question. Many variables to consider.

Big O

I am thinking trick worm when I say straight worm.  I usually use Yum dingers and would like to try something a bit different.  I am curious about the primary applications for each type of worm

Trick worms are normally straight as are most types of finesse worms but that doesn't mean that a curl tail won't work in these situations. Normally in recent times, curl tail for tx rigged presentations but then senko style worms are good for this as well. It is strictly up to the person throwing them.

Big O

I won't use ribbons when I'm fishing thick reeds.  Other than that I always start with a ribbon and if the fish don't seem to want that I'll switch to small paddles for less tail action.  

In my opinion its all preference in comparing these two styles of worms, the only thing I can think of is the ribbontail might give off more of a vibration with its curly swimming tail so maybe better in disclored/overcast days? But then again you can always add a rattle or use a dark colored worm on the straight worm, sane goes for any soft plastic.