AS Nocturnal said, both mechanics works well int heir respective game. Its pretty difficult to compare them as both game have different in nature. One has influences on the user(Just defend) and the other has influences on the opponent. ( Repel). Main distsinction between the two is one is in controlled environment and other is not.
Just defend is versatile while granting these things, nullifying any damage, life/power gain, and less gaurd stun, also allowing Air block and guard cancel. But even with all these benefits, Just defend is just an enhance blocking method that can be exploited by the opponent if respond accordingly.
While Repel/deflect rewards are guarantee a momentum switch on the opponent that can't be alter in any way if done successfully. Theirs even guard cancel that I think guarantees an vulnerable opponent as well and some invincibility. Though a none successful repel can be risky and costly, not to mention repel is limited to physical base attacks.
In the end JD has less risk and versatile in usage, but it'l only mitigate the defensive situations of the user at with limits base on the user resources. While Repel has clear distinction who has the advantage as the user has complete control while the opponent is vulnerable for set period of time.
Personally I'm fan of repel considering these.