There really is no easy answer, what is best for one person may not really work for another. They are both cool, and each one has its benefits. If we're looking at small 450-500 size helicopters we're pretty much focusing on electric as there hasn't been a reliable, 3d capable nitro model in that size. Popular helicopters in that size are the Trex 450 and 500, the Thunder Tiger mini Titan, and the Miniature Aircraft Furion 450.
To compare electric vs. nitro we really should be talking about 50 size machines. A 50 size helicopter typically uses 600-620mm main rotor blades, and if it's nitro it has a .50 c.i. (or close to it) size two stroke, glow powered, engine. There's really no exception to engine size in nitro models, some manufacturer’s engines are slightly larger such as the Thunder Tiger Redline 53, but it is still considered a 50 size engine. It's not really an option to put a 90 size engine into a 50 size airframe, not that it couldn't be physically modified, but it wouldn't be an easy bolt in deal. Some popular 50 size nitro helicopters are the Thunder Tiger Raptor 50, The Hirobo Sceadu Evo 50, and the Align Trex 600N.
An electric 50 size helicopter still uses 600-620mm main rotor blades, but usually relies on a brushless motor, and lithium polymer batteries. 50 size electric models available now are usually powered with 6-10 lithium polymer cells, and there are many different brushless motors available. Some popular 50 size electric helicopters are the Thunder Tiger E620, The Align Trex 600e, and the Miniature Aircraft Razor. These helicopters are all dimensionally very similar to nitro models. Like I said before, nitro helis are different than electrics. Obviously if you buy an electric helicopter, you won't be spending any money on fuel, fuel pumps and glow igniters, but you will need battery packs, a good quality charger, balancer, and power supply.