Mathematically, if you have your back motors mounted at 45 degrees, and they are the same size as the front, the Z-direction thrust is equal to one of the front motors. Of course 45 degrees is probably not the best angle. More like 30-40 from the horizontal.
I've been flying a tricopter for a few months and I'm in the process of building a V-Tail. I went with the tricopter over a quadcopter because I thought it would be cheaper overall - it may have been but really the cost is pretty close to the same if you are starting with nothing (like me).
I'm building the v-tail quadcopter now because I want to use it primarily as an FPV system whereas my tricopter is more for fun and acrobatics.