If you're looking for something that's hard to fly, go for the Warhawk.
You've gotta manage takeoff power, strong torque, 3 position gear/flap lever, unbalanced drag from the gear that can sometimes retract one wheel at a time, then hold down the electric hydraulic button, while resetting climb power/RPMs, and adjusting radiator flaps, then checking the gear with the manual hand pump, then resetting the gear lever...all in about 30-45 seconds.
Plus the physics of the aircraft flying through the air are incredibly realistic. You can't stay heads down for too long, or you might look out the canopy and find yourself in 30 degrees of bank by accident. Your hand is also very busy readjusting rudder trim with speed/power changes. Probably one of the hardest out there to fly.
The Spitfire I don't have much experience with, but I know it's generally easier to fly, but harder to manage the cooling, etc. I think the landing gear blocks the scoop or something along those lines, which means you can't idle on the ground for more than 5 minutes...I don't remember how it goes.
That said, I actually think the Spitfire might have more systems. You can change the propeller...there's a supercharger on it and everything. The A2A P-40 is an early Allison Warhawk, so there's no supercharger, the fuel pump is a manual wobble pump, rather than the electric booster. You could probably jot down the startup procedure on the palm of your hand.
I think mostly it boils down to what you prefer. I prefer the Warhawk, but a Brit might prefer their beloved Spitfire more.