I have done both but have found that I was far more likely to go over my daily calorie allowance than I am in my Syns.
I vote for Slimming World because it may not be faster but you will see results and still feel full, therefore your likelihood of staying on plan is greater
I would choose SW over CC any day, have done and lost a fair amount on both but always felt restrictive on CC as always measuring etc. SW becomes 2nd nature after a while x