Design preference. It's entirely possible that you (or anyone else) may simply prefer the ATIV form factor or the Lumia form factor better.
I'm up in the air. I really like the current Lumia 900 style, so I'm hoping that the 920 is just a simple evolution of that design. Also, even though some will criticize it as a "leftover" or "recycled" design, I like the ATIV style too.
In fact, I argue that recycling the designs is a good thing from an R&D perspective, because it creates a sense of familiarity for less techy users, and helps them embrace technology more easily. It's also a financially intelligent idea, since you can mass produce models faster and easier and push them to market. Just imagine if car makers released a dozen or more variations of each car model every year, which would be akin to what tech companies do now.
Anyway, at the end of next week, I will have my decision on which phone I will buy upon release... for now, I remain undecided. Honestly, I think it will come down to preference of style more than anything else. The overall tech is nearly identical, and screen size is part of that style (since resolution is already supposed to be 720p on both).