I use both depending on how and where I'm using it. Generally portrait for web browsing and such, I really like the feel especially with News in slideover view. I also find portrait is easier to hold one handed as the weight distribution is better. I tend to go landscape for everything else like if I'm using the Pencil or want to use splitview for something. Safari in landscape is too BIG on the Pro though, you see a lot more content at a reasonable size in portrait.
A lot of reviews criticized the device in portrait orientation but I think it's great. Especially when you open the keyboard and you get to type on something the size of an Air in landscape, which I'm very accustomed to, and still have a huge display area to see what you're doing - even while using multitasking. Awesome!