The main difference is the art is not divisible and, to own a valuable piece, you must have lots of money. Anyone can own a bitcoin. Thus, there is no "show-off" value of value coming from knowing you own something that other want but do not have or cannot afford. There is no significant increase in utility by owning an extra 0.1 bitcoin. So, there is a huge difference between owning an art or a limited-edition collectable item and almost infinitely-divisible bitcoin.
As for the "storage of value" argument: it could be valid if there will be zero volatility and very low (i.e. inflation-level) increase in price. By a simple mathematical perspectives, the price cannot increase faster than the risk-free interest rate (or, if we allow price to have volatility comparable to market volatility, then it cannot increase faster than the economy growth, which is equal to the stock market return minus dividend yield, i.e. less that the market return). That is, in the long-run Bitcoin must produce lower return than comparable financial instrument, otherwise it outgrow the entire economy. At that point (5, 10, 50, 100 years from now) noone will want to hold it. So, rational people will try to get rid of it just before that. Unfortunately, many people are idiots, so, some rational people are betting that these idiots cannot use backward induction. So far, that rational people making money. But they must ask themselves a question: where they are on the "smart" scale to be able to exit before the crash. Bitcoin does not produce any economics value, so, any gains received now will paid by people who will lose lots of money 5, 10, 50, or 100 years from now. It is a bubble, but it seems a very slow growing bubble.