I have both and using the curved shaft trimmer makes by back hurt. It is just too short so I go around the yard hunched over while using it.

Also, it is very difficult to get the curved shaft trimmer under low hanging trees such as oranges and limes.

Go with straight shaft, the drive shaft in the curved unit won't last on the edger due to the load. They are fine for line trimmers.

I agree straight shaft is you somewhat tall 5'8" or taller especially as suggested it you have any back problems the curved shaft won't make it any better thats for sure.