Potential gains BTC vs Silver...
(IMO Silver is the better choice out of the precious metals because of its price, dual usage as a PM and industrial, and lastly growth potential compared to gold, platinum).
Which is the better speculative and which is the better investment? Silver vs BTC?
Historical returns for commodities are terrible. Especially if you buy at an historically high price like Silver is at now. Just look at a chart for the last silver bubble in 1980 and you will see what is likely in store going forward. The volume on the most recent selloff was enormous, smart money has left this trade. I would suggest staying far away from silver, as it is likely to fall to mid-20's/mid-teens. I know many silver bugs disagree, but there are two sides to every trade, make sure you fully understand the short silver rationale before you go long. Silver may or may not bounce up a little more to 40/42 spot, but will likely fall very hard at some point in the near future.
As for Bitcoins, right now they are a great speculative pick, could easily go up a lot more, but just as easily could crash, very hard, so don't put any money in Bitcoins you wouldn't be fine losing 100% of.