The city is requiring me to install a backflow preventer as part of our renovation. Looking around our neighborhood, the Wilkins 950XL and 950XLT are by far the most common, and the reviews online seem positive. What's not clear to me is the difference between them (other than about $10). The 950XLT can obviously be installed in a vault, but our county requires them to be above ground. Other than that, any reason to go for the 950XLT?