Exactly as others have said, try each one before you buy. You wouldn't buy a car without a test drive. Your ears may like one better than the other for reasons that aren't easily explained. One may work in your market better than another with your format and audio chain.
That said, being a consultant and contract engineer, I've put more Omnia gear in lately than any other brand. Haven't heard the newest Wheatstone box yet, but would like to soon. Orban, while solid sounding, has a design and features that were state of the art in 2000. Other than a new clipper in the 8600, not much has changed. If you like the Optimod sound, it's there throughout the whole line from 5500 on up.
The BW Broadcast box with the Ariane in it is decent, though not a huge fan of their clippers.
What size market are you in? Are you competing against the big guns with seemingly unlimited cash to throw at processors or are you in a smaller market where there are still a lot of 8100's chugging along?