if I'll want a full dedicated miner, then I'll need at least a rig of 3 video cards, which I'll need to invest much more money to buy all the other hardware too. Is it more wise to spend the money on hardware and get a small steady income of BTC, or use this money to instantly buy the BTC and try to speculate to earn more BTC this way?
Both methods have risk. Over the past year or so it was true that buying hardware and mining was kind of like a hedge -- if the bitcoin exchange rate crashed or difficulty rose too fast then you could always sell the hardware and come close to breaking even or maybe you would lose but at most it would be less than 50%.
That might still be true but the risk now is that FPGAs will push GPUs out -- GPU mining might end up becoming unprofitable, unless you are getting electricity at a rate less than average (e.g., $0.11 per kWh). The problem with buying FPGAs versus GPUs is that you no longer have this stop-loss exit strategy as the used FPGAs either have no value or are not as valuable when compared to how used GPUs still hold a fair amount of their value.
So, if you have cheap electricity and tech skills then buying some used GPUs and mining might not be the worst use of funds, but there is by no means a guarantee that you'll make any profit (e.g., positive return on investment and positive cash flow after a year or so, when including the equipment costs).
As far as buying bitcoins themselves -- hoping that they'll increase in value in the future, well that is taking a speculative risk as well.
Either way -- don't put more on the line than you can afford to lose.