The older ones due to higher purity or any old one to avoid a collapse in the US dollar. I’m intrigued.
You can buy old coins easily if you specifically want them, lots of sellers and it's easy to buy.
The question is, do you want to pay the premium those often carry?
Also check your dates, a lot of order coins used "Crown Gold" or slightly lower, in order to make the coins durable.
In my opinion however, (which there are a lot of folks who disagree), modern coins from a reputable source such as the Canadian Mint or US mint and others, have very exact purity standards listed. Some mints (such as the US mint) add little bit of other materials to make the coin stable so when you drop it on the floor the edge doesn't get bent in. Batches can easily be electronically checked or spot melted and checked if you are concerned.
For example, the Canadian Mint standard coins are .9999 pure (older ones were .999), they make some special edition .99999 pure ones.
Depending on which you get the American ones are .9167 except for special editions which are in the .9999 range.
The super pure coins have to be handled very carefully as they are easily damaged.
In a situation where currency has collapsed, but there is still actual trade going on, I find it highly unlikely that anyone will give you more goods in trade for a coin based on it being older. HOWEVER your chances of smooth trading experience with a well respected mint coin that can be quickly verified based on physical characteristics, is way greater than with some unknown "rounds" or "bars."
In the event that the government once again decides to try to rob everyone of their gold coins:
1. I don't think the government will be likely to say oh, it's old keep it, at this stage in the game, they will want it all.
2. The "Good folks" are not stupid government trusting sheeple like the people in the USA were last time.
3. As explained by folks not long ago going to India and other places. Turning your gold into home made jewelry and religious idols is a fun and easy home project.
4. Lots of gold is going to be going back into the dirt till the regime collapses.
Here is a couple quick links on purity history:
en.wikipedia.org
en.wikipedia.org
Again, what in my opinion matters is that the exact composition is known, guaranteed and widely accepted to be correct.
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