A lot of Aussies mount their awning on the left side as that's the passenger side. Seems like a lot of Americans just follow their lead and put it on the left, too, irregardless of driver or passenger side. Doesn't always make sense to do so, though.
Figure out which side of the vehicle you will be getting in and out of most when camping with the awning out, and if you have or are planning on a RTT, which side of the vehicle you want the ladder on. Answering both those questions will help determine on which side the awning should be.
I'm putting my BunduEl on the passenger side of my van as that's where my side doors are and is the side I'll use most often when stopped or camping. It will also make it possible for me to park side to side with my trailer and it's BunduAwn three-sided awning and have complete coverage from my side barn doors to the galley on the trailer. You can see pics of the three-sided awning on my trailer in my post for camp photos:
https://www.overlandbound.com/forums/threads/camp-photos.276/page-68#post-188245
Additionally, I'd think most people would want to put the awning on their passenger side so if you are parked at a curb somewhere quick for lunch or repairs, etc and want the awning out, you're not opening it into the street, but over the curb.
The Bundutec awnings come in two lengths, 8' and 9'6". A lot of guys get the 8' three-sided (I have the 8' on my trailer) for Land Rovers etc. Figure out how long your rack is. Most awning brackets should go in from the end of the awning no more than 15-18" if it wraps one or both ends, as the pivot point is at the end of the length and it's important it have sufficient support to open properly and last a long time.
If you're going for a straight awning, nowhere near as much stuff to consider. There's a ton of options out there for straight awnings and a lot of options for 270 awnings, too. Take your time and consider all the features, options, and construction, etc and the one that will work best for you will emerge.
One of the great things about the Bundutec awnings from South America is that they are aluminized on top, to reflect summer sun. Made a HUGE difference in a long trip I just finished. Here's the Bundutec awning page:
http://bundutecusa.com/awnings/ Rory and Jenn
@BundutecUSA (instagram username. Though they're here on OB, not active here yet) are great to work with. I've been to their facility in Raymond, Iowa a couple times now picking up stuff.