Personally I prefer wee wee pads, they are easy to clean and simple.
However, it may be best for you to try a grass like system since your little one is already accustomed to going outside... it may make the transition easier.
RASCAL DOG LITTER BOX COMPANY, L L C HOME PAGE PETaPOTTY--A grass bathroom (pet potty) for your dog, cat, or other pet Self-rinsing Grass Litter Box for Dogs on Your Patio