Hi Consto and welcome,
In regards to purer I think you are referring to the % of silver in a coin. For coins from 1937 to 1964, in general U.S. coins actually had a higher % (90%) for silver coins than Canadian coins (80%). Prior to 1937 Canada had 92% silver content versus the U.S. at 90%.
As far as long term value that all depends on the individual coin but I can say that overall there is more demand for U.S. coinage than Canadian coinage.
If you are not familiar with your Uncles collection it may be difficult to chose a gift and also it depends on how much you are willing to spend. It would be like shooting in the dark without a little more background info. Here are some alternatives that a collector may enjoy:
Gift Idea #1 Books & Supplies
1. Charlton Standard Catalog of Canadian Coins
2. The Encyclopedia of United States Silver and Gold Commemorative Coins 1892 to 1954 (you'll probably have to pick this up used at Amazon)
3. 100 2x2 holders
4. A 10x or 20x lighted loupe
Gift Idea #2 Computer Stuff
If he likes to use his computer then Krause Publications offers a coin price guide on a set of CDs that cover world coins from the 1600's to present and costs $99 http://www.krausebooks.com/product/62197/bundles
Gift Idea #3 Coin Inventory Software
Exact Change is one of the best software packages for inventorying a collection http://www.exactchange.info/
Gift Idea #4 Subscriptions
A one year subscription to Coin World http://www.coinworld.com/coinworld-magazine/
or
A one year subscription to the Coin Dealer Newsletter "Greysheets" http://www.greysheet.com/web_order/subscribe_form.asp
Hope this helps,
Ken
