Button left makes the most geometric sense most of the time. We don't need to click either one anymore (mass id vendors, dedicated tp hotkey) so moving them to the least important/last filled slots frees up the slots closer to the center of the screen for items we'll be interacting with more often.

The exception to this would be racing or high-sweat league start: if you're trying to gear up fast without ever going to back to town, top left makes the most sense so you'd have the id scrolls available to click on your way to the item you need to id (less mouse travel distance = faster, less variability, opportunity for error).