Effect of the high impedance or low impedance of oscillospe
Your point "I know high impedance give less distortion on the waveform" is not true. The high impedance mode adds a lot of capacitance to your circuit at the point of probing, and this excess capacitance certainly can and will distort your circuit, especially at high frequencies.  You will need to consider this when deciding to use one input impedance or the other.   

In general, you will almost always want to use the high impedance mode of the scope, i.e. the \$1\mathrm{M} \Omega\$ input impedance when probing a circuit.  You will  use the \$50 \Omega\$ mode if you are connecting to an equivalent \$50 \Omega\$ ohm output impedance of a source, for example a signal generator with a \$50 \Omega\$ output or an RF signal generator with \$50 \Omega\$ characteristic output impedance, or a TX device under test that has a \$50 \Omega\$ output impedance. All transmitters operating in the GHz range will be \$50 \Omega\$ outputs.  

When using the \$50 \Omega\$ low impedance input mode, you will want to use a \$50 \Omega\$ coax too. You always want your source, line and load (the oscilloscope input) to be impedance matched to minimize reflections that will add or subtract from the true signal you are measuring. 

Most work with high-frequency will want to use the lower impedance, lower capacitance mode with a matched impedance signal chain.  

For very high frequency work, there are also so called Lo-Z (low impedance) probes that are nothing more than a short section of \$50 \Omega\$ high quality coax and a series resistor of 450 ohms, giving a x10 probe at \$50 \Omega\$.  This type of probe input is used at very high frequencies, 1GHz or more with up to 10GHz bandwidth. The benefit of this type of \$50 \Omega\$ probe is that it has almost negligible capacitive loading of the device under test, compared to the \$1\mathrm{M} \Omega\$ which has between \$10\$pf-\$20\$pF of capacitance (up to \$100\$pf when you add the length of coax).  

A Lo-Z probe such as this adds a \$500 \Omega\$ load to the point where you attach the probe , so you need to be aware of the effect of that load on your circuit at that point. 

You always need to be aware of the impedance of your probe and the capacitive load that your probe places on the circuit where you attach it, and can your circuit handle it without causing loading problems and distortion. Only then will you be an oscilloscope Jedi.