Though I prefer the upper deck, too, there are some benefits sitting in the lower deck. One of the main benefits is, the overhead compartmens are larger and you can store you hand luggage more easily.

If you have to go to the front of the (upper deck) cabin to get you hand luggage you are loosing time deboarding the plane. On TG I experienced also, that Economy passenger could leave the plane, while business passengers were still heading down from the upper deck.

Beware of the frist two seats of the upperdeck cabin, as the bins between window and seat are smaller than usual. I had difficulites to store my laptop bag there and therefore had to put it in the overhead bin (which was not that bad, as neighbour seat was free). And you have not bin if you are sitting at the exit row in the upperdeck, giving you only the small overhand bin to store you belongings.

If I could get a free seat aside on the lower deck and not on the upper deck, I would go for the lower deck.