I haven't stayed at YC but have at BC which is pretty much the same other than color scheme.
A one bedroom at BCV is so much nicer I can't begin to tell you! Tons more room, a whirlpool and seperate shower, washer and dryer, full size refrigerator, king size bed, seperate living room.
I don't know about the upgrade to a 2BR but if I were traveling with a 15 month old I wouldn't want one anyway. The second bedroom is on the other side of the living area and I wouldn't want my toddler that far away. Check the wdw website for room layout and it might help you.