Okey, interesting topic here, let me add my two cents.
First of all Parry is more of an safe option and easier to pull off for some reason and also generally more rewarding, UNLESS! opponent is a long range hero where You can't hit him with a free GB after parry, meaning You might hit him with the top light combo but that doesn't always work vs Nobushi, while light deflect will always hit them and also allows You to get instantly very close to the enemy. (Also looks cooler)
And another thing that most players don't know ( this is my secrect ) There are a number of attacks in the game that You can Deflect Heavy counter attack 100% guaranteed hit, like storm rush and basically slow attacks that make You stagger after attack. But that's not all, there's a part two to this secret, You can get a free GB after Heavy counter, how many did know that?
That's total 50 DMG+35DMG Total of 85DMG! and an following zone attack has like 50% of hitting aswell (105 dmg?). Not to mention how awesome this move is and how much fear You install into Your opponent while pulling this off.
I just wish i knew all the attacks that are guaranteed to get hit by heavy counter attack!