Bullion is bought for the metal content; rare coins are bought for collector value beyond the metal. The two markets behave very differently.
Bullion
Tracks spot price closely. Liquid. Easy to value. No specialized knowledge needed. Premiums vary 2-25% above spot. Suitable for inflation hedging and core precious metals exposure.
Rare Coins (Numismatics)
Value driven by rarity, condition (grade), historical significance and collector demand -- spot price is almost irrelevant for high-grade rarities. Higher upside in good markets but illiquid and easy to overpay if you do not know the market.
Which Should You Buy?
Most savers should focus on bullion. Numismatics is a hobby and a business, not a beginner investment vehicle. If a sales rep pushes you into "semi-numismatic" coins for an IRA, walk away -- it is usually a high-markup product.