I've seen plenty of planes with both, as well as the third, where both the leading edge (LE) and trailing edge (TE) aren't straight, instead the 25% of the wing chord point is a straight line.

In theory, the straight leading edge will have a softer, more predictable stall, and may allow a plane to turn a little tighter as a result. However, in the real word, the effect is not a large one. Instead, the straightness of the LE or TE often has more to do with construction techniques, Center of Gravity (CG), and so on.

Aerodynamically speaking the straight leading edge actually results in a wing that is "foward swept", since wing sweep is measured at roughly the 25% point of the airfoil. When the LE is straight, the 25% point moves forward as you get to the tip. If a wing has much forward sweep, then you can get in to trouble if the wing twists in flight. A wing with a straight TE and swept back LE will tend to resist this twisting problem. I'm seriously over-simplifying here, and I don't think it's enough of a problem to worry about in planes the size of combat planes.

In full-scale aviation, sweeping the leading edge back helps in high speed flight, but our models don't go fast enough for that to matter.