Earlier quoted context omitted.
I don't think you can generalize it this way, ASTs could be compiled to fast machine code, also it really depends on the solutions the algorithms come up with. The NN is bound to its number of parameters, while the Genetic's program varies in length and can become quite small if length is part of the fitness function.
Well, the accrual of "useless code" (there's a name for this that I forgot") is a known problem, but it is also something that stabilizes the learning process I don't think it's as simple as putting the length of the AST in the goal function (but it's something interesting to try). Depending on compile speed vs running speed you might be better off interpreting your ASTs
I'm generally pretty suspicious of generic algorithms; why take a random walk when you can March along the gradient towards a solution?
It might be interesting to try using GA for neural architecture, though, and gradient descent to train the network... (Though it sounds expensive.)