The one HUGE advantage the Slow N Sear has over the sizzlemonster is the water reservoir which I think is a key aspect in keeping your meat from drying out during a slow cook. I think both inventions are wonderful, but no need to own both in my opinion.
I've got a Slow N Sear, two Vortex and I just backed this. I agree there are similar uses for all of these, but in the end I'm a sucker for gadgets.