Farmers handles:
Pros:
-Easier to grip
-Nothing interferes w/ leg movement
-Because of previous two, you can load up a lot of weight. If you can pick it up, you can walk with it, so you can get get close to your 1RM deadlift.
Cons:
-One more thing to buy
-One more thing to store
-You need to buy plates if you don't have them
Kettlebells:
Pros:
-You already have them
-Useful for more than just one thing (you could use handle for barbell type stuff, I guess...but a barbell would be way better for obvious reasons)
-Allows for some level of continuity if you travel regularly
-Better for grip strength. Kind of. You could just get farmers handles and, you know, load them up heavier
Cons:
-You have to hold the bigger ones out away from your legs when you walk, or your legs bump them
-Grip strength can limit your loading
-You will never be able to carry as much with kettlebells
My two cents:
If you have the money and space, get them. You can always get some fat gripz if you want a grip challenge, and they allow you to use heavier weight, which is is a significant advantage in my mind. That being said, if you you're more of a minimalist, then just use the kettlebells. It's like kettlebells vs. barbells. The kettlebell does some things really well, and everything else pretty well. The barbell is king for getting big and strong due to the loading efficiency, but less effective for other fitness goals. Farmers handles are a specialty tool that let you carry the most weight possible in two hands. Yolks let you carry the most weight possible, period (excluding wheeled devices). Loaded carries are awesome, and if you want to get the most out of them possible, you need the specialty tools. For what it's worth, I only use farmers handles when I'm at my gym, and just use my own set of kettlebells for almost everything.
