Coaxial remote control helicopters are the easiest types of models to fly and almost all sub-$50 helicopters have coaxial rotors. Single rotor rc helicopter models are also made, but usually sell for more money. Cost aside, should you get a coaxial or single rotor heli if you are looking for a fixed pitch model? Other things being equal, a single rotor model will have better fast forward speed. This can be handy if you want to fly outdoors in wind. However, there are coaxial helis including the Esky Big Lama and Double Horse Volitation that are commonly used outdoors. With some aftermarket modifications, they can be more capable outdoor machines. The flip side is that you may have a harder time at first controlling a single rotor model. Watch out for that tree!
The best solution to the dilemma is to buy more than one helicopter, then you can have a fun coaxial model for easier flying days with friends plus a single rotor machine for when you want to let ‘er rip!