I have not owned both, but have worked with both. With help from user NE8E and NickNJen on this site, we've all worked with both. The "X" model has been deployed successfully with two birdviews, requiring a rotary encoder vs. the Birdview original reed switch kit. I use the model 7 with my Birdview, it was less expensive at the time I bought and more readily available. We deploy a different rotary encoder operating at a higher voltage, and a mechanical relay to interpret the pulses to and from the box. Nice thing about the "X" model was, if you need it for our BV conversions, the small circuit board that NE8E built with LED's is visible through the dark plastic lens on the side, so you know its operational!
In a nutshell, unless there's specific needs for your system, either should be fine!