Choosing what bullion you buy is an important decision. While larger size units of gold and silver carry a cheaper premium, there are tax considerations to be mindful of when selling.
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To determine what is the “best” bullion to purchase, you'll first want to ask yourself the following questions.
Do I want the ability to sell portions of my bullion holdings without causing a significant decrease in my overall holding totals?
Do I want bullion I can use for smaller purchases and/or trades?
Yes to #1, No to #2
Based on your answers, you're likely not concerned with small units of bullion and want to purchase as much as possible at the cheapest possible prices.
With gold you have more flexibility if you stick to 1/4–1/2 ounce gold coins/rounds or 5-10 gram bars.
With silver you have more flexibility if you stick with 5-10 ounce bars, coins or rounds.
Yes to #1 and #2
Based on your answers, flexibility is important to you and you don't want to necessarily sell a large portion of your holdings at one time.
With gold you have more flexibility if you purchase 1/10–1/4 ounce coins/rounds, or 2.5-5 gram bars.
With silver you have more flexibility if you hold a combination of 5-10 ounce bars, one ounce coins/rounds and “junk” fractional coins.
No to #1, Yes to #2
Based on your answers, you want some smaller denominations for barter, trade, etc., but aren't concerned about selling a large portion of your holdings if needed.
With both gold and silver, you will want to purchase large bars for the bulk of your holdings. You can certainly buy fractional gold, but would be better served holding “junk” fractional silver coins for any barter, trade or smaller transactions.
No to #1 and #2
Based on your answers, your biggest concern is concentrating your wealth in bullion at the cheapest price possible.
The best approach would be to purchase gold in the largest quantities possible. While you can also do this with silver, keep in mind that you will need a considerably larger area for storing your silver, due to its lower density compared to gold.