The true answer to this question depends on the purpose. If stock trading for you is simply buying, holding and selling stocks, the difference will be this-
Casinos want to make a profit and it is quite natural. This is why most of the casinos design the games in a method that the odds of getting a negative return are more. Casinos will set the odds in a way that the chances of winning in a game will always be lesser than the chances of losing. This is because Casinos are providing you service and the service is for business purpose.
Stock Market is expected to give positive return if one holds on to a number of reputed stocks for a long time. This is because investors run the stock market and investors expect a positive return from a stock and that is why they take the risk of buying. Stocks equals to the customers and their interest in product or service is paying a person who is taking the risk of investing in that particular stock.
The purpose of the stock market is people here are willingly buying, holding and selling stocks. They are taking the risks to gain some profit and this is why they take an interest in a particular stock. On the other hand, Casinos are running with the purpose of making a profit. The whole thing is under the control of an authority and they are setting up the odds. A person pays in a casino to experience the thrill of taking risks.
Now, if we take casino games like Poker in our consideration, games where a player is not playing against the house. The scenario becomes same or maybe better than trading in stocks. The skills of the player now decide the chances of winning or in other words, the chances of making a profit through investment are here dependent on the skill and luck of the player. If we consider it as a short-term investment, then gambling in casino becomes a far better option than trading in a casino
The real life casinos and online casinos offer different kinds of bonuses and offer to attract customers. Such bonus and offers are not there in case of trading stocks. These bonuses and introductory offer reduce the amount of loss one may make. In games where the player is not playing with the house, such offers make it more beneficial.