Kat ... the higher the nitro content the more powerful the engine will be ... the more powerful the engine will be the shorter it will last. Higher nitro also assists in the transition between low trottle to high ... its a lot smoother. Idling is also better with higher nitro content. 5% or 15% is just fine for everyday use (I prefer 15%) ... don't bother with 25% I tried it and love it but there is no real benefit for average flying, just higher operating cost.

As a beginner try this link ... its great ... http://www.bolly.com.au/book