Both of them are have their own risk but stock trading usually depend on world issue meanwhile about altcoin trading is really unpredictable if you're not fully analyze about it
Yes, in trading altcoins you need also to study and analyze the movement on the graph. Also you must the the ANN section here on whats the Developer are doing or their update as it may also affects the coin price. Unpredictable? Maybe yes but you can minimize your loses if you are doing your assignment.