Ray - When you say compete, I assume you mean in volume. However in bluegrass they don't really compete, because the mandolin chops for most of the time behind the banjo or fiddle, and then the banjo all but drops out altogether whilst the mandolin plays a solo, I don't know why people say I'm pedantic. Whilst I agree with previous post that many a good tune is played on an old fiddle, when it comes to volume and sound quality you tend to get what you pay for; top end mandolins and banjos produce more volume in general, because they have better but more expensive component materials (solid tonewoods, brass tone rings, etc) and you won't normally find those in your average music shop, however good their range of guitars might be.