I say brushless electric is just as challenging as nitro. It's easy to make power with an out of the box brushless setup. But once you start to mess around with different motors, rotors, tweaking the ESC and whatnot, it becomes a different story. Electric brushless is also cheaper to get started with, untill you start messing around with $200+ brushless motors $200+ batteries.

With brushless you have instant torque, whereas a nitro doesn't. So startup is a bit different for each. Other than that, I think they are both just as fun.