Kabal is extremely boring simply because he's so easy to use. I suck with Reptile, but he's easily the most fun character to use because of his juggling options. With Kabal, all you have is a pop-up combo, but Reptile's juggles are almost endless. Like others have said, Kabal is for winners, Reptile is for players.
Well ... I play only with 1 friend and I main Kano & Reptile. Although I know some U. Sub, Kabal, Scorpion (and currently practicing Kung Lao) I usually pick Kabal when I get sick of my friend beating me with H. Smoke.
So, as everyone said. Kabal for Win, Reptile for Fun!
Kabal is: carefully placed spin -> 40ish % combo (Can't do better) -> spam some fireballs -> repeat til win
Reptile is more fun. He can be agressive and a zoner (and doing half of his infinite is rewarding, his juggles plain own!). And Kano, oh boy he is awesome in your face character.
Well, at least that's how I play them
P.S. By saying I know those characters that means I don't suck horribly with them. (although compared to people here ... I guess I do xD)
Kabal is more powerful I think. Setting up good combos with Reptile other than with his pop-up dial combo without a jump punch/kick started is really hard, but that dial combo is good for run-attack. After that you can do high damage combos with the dash move, but it's tough to get on most. Kabal on the other hand has an easy, basic midscreen combo that you can start easily, even after a spin. His corner combos are pretty high and trapping opponents can be easy with him.