is bokeh is better,that helps me decide,as 500+tc can give noisy bokeh ,
This question part depends what you mean by bokeh
Going from 500mm to 800mm
from the same focus distance naturally gives more magnification, a narrower angle of view - and aperture for aperture
when NOT using a TC just over 2 stops narrower depth of field - with more blur in out of focus detail.
The plus is it easier to blur either background or foreground detail (though rarely 100%) when going from 500 to 800mm
with the 800mm prime..
The minus is going from 500 to 800mm makes it more challenging to both initially locate and then to follow flying birds.
When it comes to the shutter speed usable, the 800mm at f6.3 is better than the f8 equivalent with the 500mm plus 1.4 TC.
In physics you get nothing for nothing.
Adding a 1.4 TC is different to using a longer focal length lens.
Most appreciate a 1.4 TC looses 1 stop equivalent shutter speed.
Fewer appreciate when using a 1.4 TC the 1 stop loss of shutter speed is "balanced" by the loss of 1 stop dof instead of the 2 stops using a longer focal length lens.
Dof with a 1.4 converter is about 1 stop more than with a longer focal length lens.
You are currently using a 500 + 1.4 TC to get to 700mm equivalent.
In comparison the 800 overall will give you a little more magnification, just over 1 stop less dof and will not loose the equivalent of the 1 stop TC shutter speed.