Choosing the right stackup for 6 or 8 layers
The general idea is there exists capacitance between layers, decreasing the height (or distance between layers) increases this capacitance. The second thing to worry about is the return current, the closer these currents are the lower the inductance.

For high speed digital signals you will probably want a stack up like this

Signal (top)
Ground  

OR 

Signal (TOP)
Ground
Signal
Power
Ect  

This shows below the return currents, keep in mind that adding vias also increases inductance, which will limit the high speed or RF capability. By placing the signal and RF layers close to a ground, the easier it is going to design transmission lines, and the less impedance any trace will have. 


Source: Henry W Ott PCB layer stackup

Another thing to keep in mind is the stackup:

The Prepeg layers can lower capacitance even more and allow for even lower impedance (faster speed) traces.


Source: Dos and dont's for PCB layer stackup

Crossing planes will also be a problem, so you don't want to have your top signal layer right over a layer reserved for power planes, because they will be split and this will lead to EMI problems as the trace has capacitance to both the planes. It is better for the signal traces to have capacitance to one continuous ground plane 

With high speed digital the first option is going to be a problem especially with the top layer, the second one is much better.