I am now just simulate a OP and I don't know exactly what the load is the OP must drive. So to estimate the performance of the OP what load should be proper for me?
Since you have no idea exactly what the load is, it is better for u to simulate with resisitor and cap both. To sweep the range of load resistor and cap will make you clear about the driver strength of the opamp
The load is resistor or capacitor, it depends on the output definition of your OP. If the configuration of your OP contains follower output stage, the resistor load is sufficient. But if your OP doesn't have follower output stage, u had better connect a capacitor load to the out of OP.