What's the best unit to get if your just using it for 360?
I'm sure you've heard this before, but I'd say the biggest screen you can afford. It's more important for the 360 than side-imaging for two reasons; 1) the circular, or half circular, image of the 360 doesn't fit well inside a rectangular screen so its view is naturally smaller than your side-image view. 2) you'll be standing looking down at the 360, not shoving your face inches from the screen like you can in the driver seat.
For functionality, I see little benefit in a Solix 10/12 over a Helix 10/12. However, there is, obviously, a huge difference when you go up to the Solix 15 simply for the much larger screen, but that's a massive jump in price and
amp draw too.