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.
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?
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.
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.
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.
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.