I don't have any experience with either, but I ordered and have just now received a 0 degree Slick bag. Remember, you will not just go to the store and pick it up. There are several prior threads on this subject that you should look at. Both of the bags you are considering appear to by top rated and you will need to decide whether you want a down bag or a synthetic. I basically ended up with the same question. I will know in a month if I like the Slick as well as I think I will.
I have a Montbell 30 degree super spiral down and I love that bag. It seriously stretches and forms around you. Me being a larger guy I find it very comfortable. Can't go wrong with kifaru either. Either bag is going to be a good choice.
I have a Montbell that's rated to 15degr, but the Kifaru 20degr, and 0degr that I have are the ones that have seen the most duty recently.
Both quality bags that I think you would be happy with, both pack small, and are lightweight. The Kifaru does not have a full zip if that matters to you.
I don't have the Kifaru bag...yet. I do have the Montbell 30 deg down bag and my wife has the slightly heavier Montbell 30 deg synthetic bag (she has worn her wet/icy boots into her bag while winter camping which wouldn't be recommended with the down bag). These are by far the most comfortable bags we have ever owned (I am larger and like to move in my sleep, which the Montbell accomodates). The Montbell down bag is very thin and light, so I use the sleeping bag cover to protect it...which is nice anyway to prevent any condensation from my tarp from landing on the bag. The combination (bag + bivy cover) is very light weight and functional.