The Boss difficulty is about the same in both games, however, DSI has a far less linear world design leading to its own unique challenges. The world also feels more realistic and cohesive in comparison to how sectioned off DSIII feels. The combat is also slower than DSIII, overall it's still a phenomenal game with some unique challenges. If you enjoyed DSIII then you should at least try DSI. Some of the best bosses in the series are in here (ie. Artorias, O&S, Gwyn, etc.).