ultrasonic 1 transducer vs. 2

I've been a solid user in these since I've tried them. Their capability has been pretty close to their performance info at :

http://www.maxbotix.com/Performance_Data.html 

And I much prefer having a micro directly read an analog voltage or a pulse width, rather than use I2C, though others may prefer the query-response method. One thing that was a little strange on the Maxbotix sonars, was the serial connection. I think it needed to be inverted, but never used that interface method.

On using one transducer, the old Polaroid sonars (now Senscomp) used active "blanking" to quiet the transducer after a ping. These probably have something similar. 

As to cost, even though they have one less transducer, there still is a PCB, a microcontroller (Microchip PIC) and support circuitry to pay for, just like the dual types. The PCB cost is still the pretty similar at this size, even though it's a smaller board (don't know if it's more than 2 layer, but probably not).  The micro cost is probably the same.The sonar transducers are probably $2 each, so not a great impact on the total price.