With a simple is better philosophy, a panel loader loses hands down. I like a panel loader for travel, but for bare bones UL hiking, the top loader is simpler, lighter, and allows multiple use of my sleeping pad. I do like to have a top pocket and side pockets for access to commonly used items like rain gear, maps, compass/GPS and snacks I use a Golite Speed now). Any clothes I might want to add or subtract are in an outside pocket or at the top of the main pack bag. So why would I want a panel loader on the trail?
As to zipper failures, the last thing I want to to have is the main closure on my pack exploding gear down the switchbacks! Any panel loaders I used had (and needed) compression straps---usually four-- and thats a lot of hardware, expense and complexity. In fact the compression straps and zipper probably exceed all the other pack hardware combined, let alone assembly time and increased opportunity for warranty issues. Drawstrings are easy to make and repair in the field.
When I travel, panel loaders are great, as I can toss it on the bed or luggage stand in the hotel, unzip the panel and take clothing out. It also allows packing my clothing with minimal wrinkling. I travel light, but still go for heavier check-in luggage due to Baggage Gorillas. The idea of a silnylon pack on a baggage line makes me shudder. An Eagle Creek Cordura panel loader/convertible pack is perfect for me there.
Considering the same on the trail, a panel loader would only be of advantage in camp, but I end up taking out everything in the main bag anyway-- the sleeping pad comes out, and my sleeping bag is on the bottom, with bags of clothing, food, and kitchen stuff on top, all of which I need to get into. After dinner, the kitchen and food bags get hung in a tree and my pack and clothing go under my feet if it is cold. I guess a panel loader would make a stable pillow.