After the last couple of days and many hours behind the seat this will be my recommendation to my customers. I have run the G1 and G2n mega side-by-side and the G2N mega and Onix.
In the way HB has always had great SI they now have a great Si/Di machine at a great price. The dedicated Di element in the mega units makes this line a complete unit that honestly blows away everything else in the price range and many of those above it. I do have customers that already have Helix Gen1 si and di units both at the console and for them I can't see the additional investment at this time. Unless of course they want the bluetooth features or just like new toys! TheG1 H12 si and H10 di units we ran next to the Mega 10 pretty much held their own. Were the mega images sharper?, sure and they were very impressive but for the money my folks have already spent I think waiting won't hurt them at all.
Now for the mega vs Onix. For me there are just to many features on the Onix that I really enjoy using to leave for the mega. The Onix is still king of the Di with the Mega and the G1 H10 being close. The mega Si images are sharper but honestly if you are running an Onix it's close, very, very close with the Onix at 800Khz. Despite all of it's issues the Onix hasn't been knocked off the top just yet when it comes to imaging and details.
So FWIW there is my opinion for now. As usual everything is subject to change as I spend more time with any one unit.
I was also running another brand in the same price range as the Mega 9 and if you are looking for a unit in that price range just buy the Mega and be done with it.