The main difference of the Jabra Evolve2 65 Flex vs the Jabra Evolve2 65 is:
- Foldable
- Hideable mic boom
- More lightweight and lowprofile
- Better noise canceling microphone
If you need a hybrid headset that can be used for work calls and also personal audio with the hideable microphone, the Jabra Evolve2 65 Flex is best to go with. Otherwise, if you only need a headset for calls and meetings and you're on a tighter budget, the Jabra Evolve2 65 will be more cost effective to give you a good quality wireless headset.
That being said, for about the same price you might as well go with the Jabra Evolve2 55 as it's newer and an upgrade in just about every area. You'd think the Evolve2 65 is newer but the Jabra Evolve2 55 is actually the newest model of the two.