I think there's pros and cons to both.
Pros to points system: It really is more based off your skill and KD. If you're a better awper, you'll get higher on the leaderboards, that's just a fact.
Pros to kills system: If you're a weaker player, you still have incentive to care about the rank system. If you're a weaker player, seeing the next 10 people and saying "I'm gonna make it a goal to pass them" is an incentive. This does tie into if you play more, your rank is higher.
Cons to points system: it can be incredibly discouraging if you're not a great player. As a competitive person, i care about my rank, but if I'm having a bad day or my aim is off. where I would use to come play and just have fun in the awp server - I might not, because I don't want to lose those precious points. Not to mention, the way points in general work is often times at higher ranks you have to keep ridiculous KDA's in order to maintain when you gain 2 points/kill and new players (who are okay at awping) take 30 in a single kill. Unfortunately point based systems have to follow this rule otherwise good players would never have a ceiling.
Cons to kills system: The same discouraging thing can happen if you're a good player and if you're new. If you're new, it takes much longer to rise in the ranks because you're behind so much already. It means you have to out-"killscore" all the people you're chasing AND you have to play more than they do.
At the end of the day, though, they have their pros and cons. Kill based leaderboard metric made me enjoy the server on a more casual basis, and personally I'm on the side where I won't be too excited to join the server if im having a bad aim day and I'll probably end up leaving the server if I start to tilt.