Flexi rods elongate the curl and gives a more 'ringlet' look. Perm rods create a tighter, shorter curl. I like flexi's over perm rods because they are less stressful on the hair and they allow more air to cover the hair, which reduces drying time.
I need to do me another rod set as I think about it!
Here's my flexi rod tutorial. I was in a rush when I did this style, but it came out pretty well!