It's about time someone made a longbets prediction, but I don't think the time period is long enough for it to be fair.
The problem is, gold and silver are in bubbles and will probably crash in the next few years, the stock market is crashing now and (unless there's hyperinflation) the USD doesn't change that much over a period of a year and a half. In 2013 the mining reward will halve, and we don't know how that will influence BTC. A year and a half from now, governments probably won't have had a chance to attempt to attack BTC yet (remember, bureaucracy).
A period of 10 years would be better in my opinion, because it won't be as much influenced by short-term events and will show the long-term trends better. Also, it will give governments ample time to try to attack, so that we can see exactly how well BTC will weather the attack.