I have not transited at either airport, only arrived/departed, but generally speaking SGN would be busier. It might also have a bit more by way of amenities and facilities airside but neither airport has much in that respect. There's a VN Lotus Lounge at both. SGN, being the bigger airport, might be a better lounge but I'm not sure, and I would not expect too much from either one.
I would probably pick HAN, all else being equal, but I also would not use this difference as the deciding factor on which flights to choose.