RE: Heavy or Light
The heavy model won't be able to sustain speed very long in level flight. For those of us who don't see much value in purely gravity assisted speed records, the European format is the definitive way to measure a prop driven planes' worth.
If you do your best to scratch build a sleek model out of the best materials and make it just strong enough to withstand the forces of flight, the end product will probably be a speed plane that you wish could have come out even lighter if some things were done differently. Building heavy is easy, it is something that anyone can do.