I have used all 3 (fabric, boom and HH).
I never really had an issue with fabric handles, then the javelin came out and I have the boom with 4.0 and 5.0 sizes.
positives are how rigid it feels, it helps in tacking toe side and heel side as you can always slide your hands into position.
For gybing it makes it sooo much easier it's like cheating and it also helped me boost jumps.
The only real downside is the added weight, and they don't flag as well when surfing a wave.
I have had 5-6 sessions on the hard handles (4.5 and 5.5, have not used the 6.5 yet) they feel absolutely amazing, nice and rigid with
a locked in feel, you can switch to pistol grip for flagging and the boosts are very fun.
I made a video on the Javelin 5.0 and 4.0 and next couple of days I will post the Hard Handle footage.