The EZ and the VA do the same thing. Difference is that the EZ includes boost control and has three setting to determine when the fuel is added. The VA fuels immediately and the EZ fuels depending on where you have the jumper set. A VA with PS boost module performs the same as an EZ.
The VA connects directly to the CAN bus of the injection pump and the the EZ connects to the MAP sensor. I have not checked pricing in a while but last time I looked the EZ was cheaper than a VA and PS boost module combined.
As far as performance, if you tow, I love this setup with 275 injectors. The VA or EZ add power down low all the way up to about 2k rpm where it starts to taper off towards just above stock fueling. The 275's really start to come alive around 1900 all the way up to redline. With the two, this truck just plain pulls it's butt off.
My mileage did not change with the VA but EGT's were LOWER than stock. After the injectors, EGT's increased to stock levels, and my fuel mileage increased almost a full two 2mpg.
The only thing I don't like about the VA is that since it fuels right off idle, you will get smoke with no boost, meaning if you hammer it, you will puff black smoke until boost builds. This would be amplified if you wanted to add another box (PE/Blue Chip/TST/etc. ) where as you could set the fueling to be delayed on the EZ until boost is present.
Hope this helps.