I vote for convo- Deaf owned business and great services. I used to have Purple, but honestly I didn't like the variability in interpreters. A friend of mine works for Sorenson and swears they are great, but I am happy with convo.
I'd love to stick with Convo but their GUI isn't good for me and my vision . Using ZVRS (Z70 and Z5) right now. Sorenson is okay but I don't like the idea of using my TV as a dual purpose TV (or if I even can...). At least the PC version has a dark theme (Their VP2 doesn't have it). The few times I used Sorenson there was a variety in interpreter's skills and demeanor- one was practically sprawled all over her chair while I was on hold and not exactly dressed appropriately either. (!). There's always gonna be a range of interpreter skill/comfort level.