As far as 'common currency vs. different coins, as long as it fits your character, it will fly. I use Peace dollars and Canadian silver dollars busking, and nobody has said a word about it. The coins end up in the spectator's hands at the conclusion, and they can withstand scrutiny. People seem to think that common coins would be gaffed as well - maybe even more often. Why would you ruin an old silver dollar (that is most likely valuable), as opposed to a loonie?
Also, context is a consideration - I'm a busker, I'm in my forties, so it just fits that I would have some nice props, including neat old coins. A lot of my patter is about stuff I've picked up here and there, including these old coins. If I was in a bar and doing something casual for friends, I'd borrow the coins. If it's for a show, then I bring our the nice props. If I were younger, I could see how the coins may look out of place. I have not found any real difference in terms of people thinking the coins were gaffed between peace dollars and toonies. They think the coins are gaffed until/unless they're examined.
I have giant hands, and toonies just can't be seen in them. Especially if there's any kind of crowd building. Also, vanishing a big old Peace dollar looks more impossible than vanishing something smaller.
re:canadian halves
Just visit a coin dealer - the common dates are not very high above their face value. The 1967 coins have a howling wolf on the reverse, and look rather cool (they're silver as opposed to clad copper, so the '67's are pricier). You might even be able to get some through your bank, as they're still circulating currency.
As for 70's coins looking dirtier, it's because of the composition. Up to 1964, US currency was minted in 90% sliver, 10% copper. After that, they went with a copper core clad in nickel. In Canada, I believe it was 80% silver, 20% copper until 1967.
Also, "soft coins" (or 'slicks' to coin collectors) means "worn down." Older silver coins that were in circulation a long time have the relief worn down, so that the faces become slick. They're called soft coins as the faces can slide against one another quietly. Ask your coin dealer if they have any "junk silver" you can look at, and those will be the soft coins.
To clean coins, ketchup, vinegar, or lemon juice all work.
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