The Kindle Oasis's screen is one inch less than the Kobo Forma but they are both extra wide. Kobo, it appears, is mimicking the Oasis's focus on auto-rotation and landscape reading. I prefer the Kobo Aura One's dimensions - it is near identical to the iPad Mini's dimensions and has the feel of reading a medium-sized A5 notebook.
I don't think the extra width will affect PDF landscape reading as it does with the Kindle Oasis. The problem with the Kindle Oasis is Amazon's software and its limiting three size margins settings. This means the text on some PDF documents, even with the narrowest margin settings, appears too large (I haven't tried installing KOReader on the Kindle Oasis as you need to jailbreak the device). KOReader, in contrast, is an easy add-on to install on Kobo devices and supports far greater features to manipulate the viewing of PDF documents.