It depends on how you feel about smaller bags. I personally like the small size more but that's because I normally only carry phone,key, compact wallet, and a lipstick. The medium would be too big for my everyday needs. I also recommend wearing a medium and a small side by side to compare how it looks on you. The medium size was too big for my frame so that's also why I went for the small.
As for whether to buy the display piece, it also comes down to how you feel. If you feel iffy about it, I'd wait. Since it's only the beginning of fall season, I feel like the store may get more bags in the future. I got the display piece since it was the only one in the store. The SA reassured me they had only got the bag 2-3 days ago and I checked the bag thoroughly before I felt comfortable buying it. I hope this helps you with your decision