Really i don't see any advantage buying physical bitcoin designs that are made by some persons.
The 100 trillion dollar Zimbabwean note was worth around 25c when it was an actual currency. Now it's obsolete they go for $10-20 all day long on Ebay. Their government should get in on the act.
Even if BTC dies then it will never be forgotten and a Casascius coin in particular will always be of interest to collectors. They ain't making any more and it is THE image for Bitcoin in countless media articles. All the other ones ain't worth shit in the long run IMO but those are cool little hedges.