A biplane is a good way to put a lot of wing area into a compact space. DARock is right that if the two have the same wing area that the monoplane will be more efficient. The losses from the 4 tips DO matter.

But if your competition event is span limited but not area limited then going for a biplane will allow you to have more wing area and thus a lighter wing loading and thus a slow sink rate and thus longer duration from any given altitude. But it'll only help if you can keep the weight of your biplane down to very close to what a monoplane would be.

Super light indoor duration competition style materials and building techniques are what I suggest. For example the world indoor microfilm class models are a 50 cm wingspan and have a minimum weight of 1 gram. Modelers had to overbuild to get the weight UP to that minimum by going for rather large wing chords or even biplane designs. Now I would not expect you as a beginner to manage to build this light but I'd suggest you google for "indoor microfilm model airplanes" and get a feel for what they are managing to do.