Which is better: Invest my Bitcoin on legit mining sites and purchased power before the halving because it is currently cheap or save my bitcoin and trade it after the halving because its value might be doubled?
Well i have to say that your first plan to invest in mining was good when price was 400$ and lower, but now when price is 750$ it's bad plan
Now i think that you should just buy some more bitcoins and keep them in your wallet, and after price rise sell them and you will get profit
Yes, You certainly need to keep hold of your BTC until after halving. The price will once again increase soon enough. Just wait.