Difference between Dense and Activation layer in Keras

As @MarcinMożejko said, it is equivalent. I just want to explain why. If you look at the Dense Keras documentation page, you'll see that the default activation function is None.

A dense layer mathematically is:

a = g(W.T*a_prev+b)

where g an activation function. When using Dense(units=k, activation=softmax), it is computing all the quantities in one shot. When doing Dense(units=k) and then Activation('softmax), it first calculates the quantity, W.T*a_prev+b (because the default activation function is None) and then applying the activation function specified as input to the Activation layer to the calculated quantity.

Using Dense(activation=softmax) is computationally equivalent to first add Dense and then add Activation(softmax). However there is one advantage of the second approach - you could retrieve the outputs of the last layer (before activation) out of such defined model. In the first approach - it's impossible.